Dominic Thiem capitalized on two of his four chances to break John Isner's serve, and that was all he really needed. Thiem was able to break the tall American on his first chance in the second set and then once more in the third, defeating Isner 3-6, 6-3, 6-4 on Friday to reach the semifinals at the Madrid Open for the fourth straight time.
“He is one of the best servers in history, especially here in the altitude. His serve is bouncing so high and it's so difficult to return,” Thiem said.
“I really stayed focused, with a good fighting spirit. Against guys like him, a few points decide and that's when I saved the break points in the beginning of the second set.”
Thiem saved four break points in the fifth game of the second set, then broke Isner's serve in the next game to take the match to a third set. The Austrian squandered another break chance in the first game of the deciding set but capitalized again at 4-4. He then served out the match.
Thiem will next face either second-ranked Rafael Nadal or Alexander Zverev, who play later Friday. Thiem lost to both players in his two final appearances in Madrid — to Nadal in 2017 and to Zverev in 2018.
Isner served more than 100 aces in his four matches in the Spanish capital, where the high altitude adds speed to the balls and makes the clay courts faster.
The American had 18 aces on Friday and 29 in his three-set win over sixth-seeded Andrey Rublev on Thursday, including two consecutive to close out the match. He said he was outplayed and only stayed in the match thanks to his serve.
The 39th-ranked American had lost in the round of 16 in his last two tournaments, with his other quarterfinal appearance this year coming in March in what was his first tournament since last year's French Open.
Thiem, last year's U.S. Open champion, was coming off a break after a slow start. The fourth-ranked Austrian hadn't played since March. The Madrid Open was not played last year because of the coronavirus pandemic.