Eala had to hold her nerve in the opening game of her match against Sawangkaew, staving off four break points as she quickly started to settle in to the contest.
It wouldn’t take her long to go on the offensive herself, claiming back-to-back breaks of her own before serving out the set at the second time of asking.
Eala kept the pressure on in the second but had to be patient before seizing her chance. Another brace of breaks at 2-2 and 4-2, all but sealed the win for the young rising star.
She is still yet to drop a set this week and has only been broken three times across three matches, in what is her first grass court tournament of the year.
In the next round, she will play Rebeka Masaravo, who battled past Mary Stoiana in three sets earlier in the day.
Masarova also knocked out reigning HSBC Championships title-holder and third seed Tatjana Maria in the second round.
The other semi-final will be fought between fifth seed Nikola Bartunkova and Ashlyn Krueger.
The Lexus Birmingham Open semi-finals will take place on Saturday 7 June. Live coverage of the WTA matches can be viewed on WTA Unlocked and ATP Challenger matches will be streamed on Tennis Channel.