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FLUSHING MEADOWS, N.Y. — When Carlos Alcaraz walks onto Arthur Ashe Stadium Friday afternoon for his U.S. Open semifinal, his every move will be under the microscope. Not from the 24,000 fans in the stands, or the millions watching at home, but from the man on the other side of the net.

Novak Djokovic studies his opponents like a criminologist poring over forensic evidence. “Even though there is no physical contact in tennis, there’s still a lot of eye contact,” he said in an interview with 60 Minutes a couple of years ago.

“When we are changing ends, when we’re sitting on the bench, and then the big screen shows how he drinks his water. And then I’m looking at him. How is he drinking water? Is he sweating more than usual?

“Is he breathing deeply or not deeply? And then I look at how he’s communicating with his team. You have all these different elements that are in play that really affect the performance and the game itself.”

Alcaraz is intimately familiar with this process. At January’s Australian Open, it dragged him to one of his most painful Grand Slam defeats. Alcaraz and Djokovic met in the quarterfinals, with Alcaraz outplaying Djokovic in the first set before and after the 24-time Grand Slam champion tore his hamstring.

But once the second set began, Alcaraz let Djokovic’s injury — and his body language associated with it — scramble his brain. He stopped focusing on his own body language and became obsessed with his opponent’s. A tennis cliché that players love is that they always focus on themselves, but when it comes to body language, masters of the art do the opposite — just not as much as Alcaraz did that night.

After he had lost in four sets, Alcaraz admitted that he had become distracted by his opponent’s physical state, while Djokovic noticed how preoccupied Alcaraz was with what he was doing. “I felt that he was looking at me more than he was looking at himself,” Djokovic said.

Body language is a factor in every tennis match, whether or not one of the players involved is managing an injury. Fist pumps and slumped shoulders; shadow swings and lowered heads. A player’s demeanour and movements communicate what they are feeling to their opponent, but also to themselves, offering positive reinforcement and pep or adding to the already heavy weight of an important tennis match.

“It’s working in two directions. It’s working externally towards your opponent but also internally towards yourself in order to take control and take charge of how you feel in any given moment,” Dan Abrahams, a sports psychologist, said during a phone interview.

Alcaraz has developed his mastery of this dynamic since his loss to Djokovic. Once almost always upbeat with a puppy-dog smile, or looking confused by a downturn in form, he now mixes the exuberant with the hyper-focused.

“We’re playing two kind of matches at the same time,” he said in his news conference after beating Jiří Lehečka to set up a possible semifinal against Djokovic, who later beat Taylor Fritz to confirm their ninth meeting. “It is the match of playing, the points and whatever, and the match behind the scenes, let’s say.”

“Doesn’t matter if you feel exhausted, you feel really tired, you feel like you cannot keep it going. If you show the opponent you’re fresh, you’re able to play two, three, more hours, to play long rallies, it’s giving him the mindset of this is going to be really tough.”

“So for me the way that I walk between points — with attitude, like I’m fresh, I’m really good physically — I think it’s really important to show the opponent that they are going to sweat a lot, and they are going to have to run a lot if they want to beat me.”

This felt like the battle lines being drawn for his match with Djokovic Friday. It was reminiscent of Andre Agassi saying in an interview last year that “as far apart as you are on a tennis court, you can actually feel the other one very intimately.” Agassi was an even keener reader of his opponents than Djokovic, picking up on little tells, like Boris Becker’s sticking his tongue out inadvertently revealing where he was going to serve.

From Alcaraz’s comments, to Amanda Anisimova focusing on positive affirmations in turning her 6-0, 6-0 Wimbledon final defeat to Iga Świątek into a quarterfinal win, to Naomi Osaka staying focused while her quarterfinal opponent, Karolína Muchová, dealt with a left thigh injury, the past few days of the U.S. Open have seen those “two matches” that Alcaraz describes play out on one of the sport’s biggest stages.

After beating Świątek Wednesday, Anisimova went straight to body language to explain what she had improved.

“I think that was something I was really lacking in the Wimbledon final,” she said in her news conference.

“I wasn’t really showing much, or my attitude and my presence wasn’t really there. So I think that’s also something that I learned that I need to do better at.

“(Today) I feel like I was really supporting myself, which in turn, also helped me play better.”

Showing the outward emotion required to intensify positive reinforcement is not natural for every player. As Alcaraz and Anisimova’s active focus on it shows, positive and negative body language — something which is so personal that it can appear instinctive — is also something that can be exaggerated to meet a certain situation, or even taught.

Brad Gilbert, a commentator for ESPN, noticed that Taylor Fritz was fist-pumping more than he had “ever seen” during the third and fourth sets of his quarterfinal against Djokovic. Fritz, playing in front of his home crowd, sought to harness the energy, but he is not disposed to doing it constrantly.

“Maybe he could have done that from the beginning. But obviously, if it’s not in your DNA, it’s hard,” Gilbert said.

Kevin Anderson, the naturally reserved former U.S. Open and Wimbledon finalist, practiced fist pumps with his sports psychologist to try and help energize himself on court. “When you’ve played a good point, sort of acknowledging that also has a lot of positive effects that increases your confidence level,” he said after reaching the 2017 U.S. Open final.

A day after the Fritz vs. Djokovic match, Lorenzo Musetti was jumping and down while 5-0 down in the first set against a merciless Jannik Sinner. Iga Świątek, who has worked with a sports psychologist for many years, has recently made a conscious shift toward expressing more emotion on court rather than holding it in.

And Djokovic, who vents to the umpire, whips up the crowd and jaws with his box one moment, often with expressions of disbelief or rage mixed in, before blowing kisses and pumping his arms the next, always seems to remain in control.

Positive body language can influence production of performance hormones like testosterone and adrenaline, while negative or angry body language can increase production of cortisol, the stress hormone. “But anger and the expression of anger can be helpful on the tennis court,” Abrahams said.

“Anger, when you express it, can really focus the mind. It can raise your feelings of energy and subsequently intensity and it can heighten the sense of purposefulness, proactivity, and energy.”

Aryna Sabalenka’s team follows this principle. The world No. 1 is one of the most demonstrative players on tour, every emotional contour of a match etched on her face and in her body language. Instead of suppressing this instinct, her team developed a mantra: “don’t fight it, don’t feed it.” Sabalenka often expresses anger or frustration even after hitting absurdly good shots, especially if she is down on the scoreboard or coming back from a break of serve, but she also does it when winning.

One legend of recent tennis history is held up as the body-language gold standard: Rafael Nadal.

Gilbert remembers being at the 2006 French Open final, sat next to Nadal’s support team, and seeing the Spaniard aim a fist pump in their direction. He was 5-0 down in the first set. “That attitude is literally the rarest of rare,” Gilbert said. “Because more often than not, when you watch tennis you absolutely know who’s winning and who’s losing.”

But there’s also the intimidation factor. In the 2022 French Open final, which Nadal played with one foot numbed with injections, he faced Casper Ruud. In the bowels of Court Philippe-Chatrier, Ruud stared ahead, asking how long it was until they started. Nadal was jumping around and running, shadow-swinging with fury. The match looked over before it had even begun, and Nadal duly won in straight sets.

“He was a nightmare before you played him,” Nick Kyrgios said on talkSPORT this year. “He would get his racket and swing it really hard in front of you and try really hard to intimidate you. And it did work. You’re sitting there maybe listening to music, and you just hear this racket zooming and he’s jumping around. He did make you feel a bit nervous.”

Abrahams shows his clients, even ones from other sports, footage of Nadal.

“He’s using his physicality to mentally dominate himself and his opponent so in that respect he’s just this wonderful model,” he said.

Nadal extended this routine to the coin toss. Mental training coach Rob Polishook wrote in his paper, “The Mental Game: Broken Rackets – Bad Body Language”:

“Rafael Nadal’s pre-match ritual during the coin toss is the ultimate example of positive body language. Nadal bounces around feverishly before sprinting back to the baseline, making it clear to all that he is eager to begin the match.”

After winning the 2017 U.S. Open, Nadal said in a news conference: “No, I was not calm. I was nervous, but all the body language that is not in a positive way is stupid to make it, because it’s going against you.” While playing at the Laver Cup a couple of years later, Nadal shouted “Not one negative face” at team-mate Alexander Zverev.

“It is one of the things that I tried to do all my life, that the body language helps me, not go against me. Because it is one of the things that depends just on me, not on the opponent.”

Not all players try to be totally impervious to what is happening at the other end. Sam Querrey, a former world No. 11 and ESPN analyst at the U.S. Open, said that his focus on himself against his opponent was about 80 against 20 per cent. He tried to avoid looking at Djokovic, who he beat in a famous Wimbledon upset nine years ago.

“Novak’s got good body language,” he said. “It’s intimidating. I don’t need to look, I know that’s what he brought every single time.”

He said it was the same with Nadal and Federer, who was generally undemonstrative but would occasionally let out a roar of celebration, and a fist pump. The rarity of it gave it even more impact, but sports psychologists tend to focus on consistency of demeanor — exactly what Djokovic didn’t do against Alcaraz in Melbourne earlier this year, which is why it so flummoxed the Spaniard.

“Being consistent in your responses to winning and losing points can be helpful, as changes in your demeanor are often quite revealing about your psychology,” said sports psychologist Marc Sagal, via text message.

“Of course, there are times when it’s smart to purposely project a kind of energy or aggressiveness to signal your competitive spirit and readiness. Managing even the smaller signals that you might be giving off, like how you drink water, how you look over at your coach, how you cool off, are all potential tells and so it’s smart to keep these as consistent as possible.

“Once you’ve gained control over these aspects, you can then be more strategic in how you strategically manipulate your own behavior to psych out your opponent.”

It’s this manipulation of which Djokovic is a master. He tends to be more effective when he is demonstrative with his irritations than when he tries to get the crowd onside. Against Fritz on Tuesday, he relished blowing kisses towards some of the more hostile American fans. And from the moment he had won that match, he went into provocateur mode ahead of facing Alcaraz.

“We know that they are the two best players in the world,” he said in his news conference, referring to Alcaraz and Jannik Sinner.

“Everybody is probably expecting and anticipating the finals between the two of them,” he said. Then, stroking his beard, he added: “I’m going to try to mess up the plans of most of the people.”

Alcaraz knows what that feels like, having lost four of his last five meetings with the 24-time Grand Slam champion.

“I know I played a lot of times against him,” Alcaraz said. “I really want revenge.”

(Top photo: Clive Brunskill / Getty Images)