The Greek had beaten Benoit Paire the night before, 6-3, 6-1, in just 68 minutes.
He’s now reached the quarterfinals or better at his last four tournaments in a row, having made back-to-back semifinals at the Australian Open and Rotterdam and the quarters in Marseille last week.
Tsitsipas and Isner's head-to-head going into Wednesday night couldn’t have been more even—not only had they split their four previous meetings, 2-2, but seven of the nine sets they’d played had gone to tie-breaks. And in 103 service games in those four matches, there were only two breaks.
That all went out the window in their fifth career meeting in Acapulco, as Tsitsipas broke Isner in his opening service game and rode that break to the first set, then broke another two times as he opened up a 4-0 lead in the second set. The two players then held from there until it was all over.
“Things worked out pretty well,” Tsitsipas said. “I had good returns in the games today, and that was very important—you don’t want to get into playing tie-breaks with these guys, it can get dangerous.
“Coming into matches like this I try to work on my return a lot.”
Tsitsipas’ return wasn’t the only thing working in the match—he hit almost eight times as many winners as unforced errors, 23 to 3. The three mistakes? Two missed backhands and a double fault.
Isner, meanwhile, didn’t even play badly, finishing the match with 16 winners to 17 unforced errors.
Up next for the No. 1-seeded Tsitsipas is the No. 7 seed, Felix Auger-Aliassime, who bounced back from 3-1 down in the second set to beat Sebastian Korda earlier on Wednesday night, 6-3, 6-4.
Tsitsipas has beaten Auger-Aliassime the last two times they’ve met, but they’re 2-2 overall. The Canadian’s only two Top 10 wins are actually his two wins over Tsitsipas, beating him at Indian Wells in March 2019 (when he was No. 10) and at Queen’s Club a few months later (when he was No. 6).
Meanwhile, No. 2 seed Alexander Zverev rallied from an early 3-1 deficit to beat Laslo Djere, 6-4, 6-3, and will next face No. 8 seed Casper Ruud, who beat Dutch qualifier Tallon Griekspoor, 4-6, 6-3, 7-6 (3).
“Today what showed the most is I got better as the match went on,” Zverev said after closing out Djere. “This is something very important for me. When I get into a rhythm, I feel very comfortable.”
It was a mixed bag for the other seeds, with No. 5 seed Grigor Dimitrov advancing but No. 4 seed Milos Raonic losing 10 of 12 games from 4-2 up in the first set to fall to Dominik Koepfer, 6-4, 6-2, and No. 6 seed Fabio Fognini also falling in straight sets against another lefty, Cameron Norrie, 6-4, 6-3.
Rounding out the final eight will be Italian teenager Lorenzo Musetti, who followed up his first-round upset over No. 3 seed Diego Schwartzman with a 2-6, 6-3, 7-6 (1) win over Frances Tiafoe. Tiafoe led 3-0 in the third set of that match before Musetti came back to prevail after two hours and 43 minutes.