The pair traded blows but ultimately couldn’t be separated until the final set tiebreaker, which Shelton won 10-7.
The epic now stands as the latest finish in the history of a tournament increasingly known for its overnight duels.
That honor had previously been held by another Alcaraz match, against his rival at the top of the sport Jannik Sinner, which finished at 2:50 a.m in the 2022 quarterfinals. That victory helped Alcaraz, then 19, on the way to his first U.S. Open title.
He’ll now set his sights on regaining the form that made him the sport’s leading light.
Three of Tuesday’s four singles quarterfinal matches went to match tiebreakers, the day starting with Aryna Sabalenka edging Wimbledon champion Linda Noskova to keep alive her hopes of a third straight U.S. title.
Tiafoe then outlasted fellow American Alex Michelsen, staging repeated comebacks to set up the semifinal with Shelton.
Thousands of fans stayed to the gripping end, with both Shelton and the popular showman Alcaraz enjoying loud support in Arthur Ashe Stadium.
“It’s incredible,” Shelton said. “I always say that this is the greatest sports city in the world, I say that this is the greatest stadium in the world. I’m just hoping that my best friend and my sister over there don’t have to be at work at 6 a.m. in three hours.”