American Steve Johnson upset fellow American and second-seeded John Isner 6-3, 5-7, 7-6 (3) in two hours and 14 minutes on Thursday to advance to the semifinals of the ASB Classic in Auckland, New Zealand.
After the seventh-seeded Johnson controlled the opening set with his strong service game, Isner drew even by winning the second set, breaking serve at 5-6 when Johnson hit an errant drop shot.
Johnson smashed his racket into the ground after losing that set, but he regrouped to win the final set.
"I felt like I played a better set than me losing 7-5 so I took that with confidence heading into the third," Johnson said.
Isner netted a forehand volley to bring up three match points for Johnson in the third-set tiebreaker, and Johnson hit a forehand net-cord winner on the next point to close out the match.
"Credit to Steve, he stayed the course and certainly played better at the end of the match than I did," Isner said. "I just missed a few easy shots. The first point of the tiebreaker, that sort of set the tone how the end of the match went -- it was a lazy volley on my part. I needed to be more alert."
Johnson's semifinal opponent will be fourth-seeded Jack Sock of the United States.
Sock and Johnson won a bronze medal together in the doubles at the Rio Olympics.
"I feel like I know what he's going to bring and I'm sure he knows what I'm going to bring. So if we do play each other there's no secrets," Johnson said.
Sock beat Jeremy Chardy of France 5-7, 6-4, 6-3 in two hours and 10 minutes.
Sock hit 12 aces to Chardy's 14, but Chardy's ability to hold his serve in difficult situations kept him in the match.
Chardy saved 21 break-point opportunities in the match.
"I've played matches with a bunch (of tiebreakers) but that was crazy," said Sock. "It was a credit to him but eventually I got some."
Chardy saved all 11 of the break-point chances against him in the first set, while Chardy converted his only break-point opportunity against Sock.
Sock finally broke Chardy's serve late in the second set to even the match and raced out to a 5-1 lead in the third set before finishing it off.
In another quarterfinal match, Joao Sousa of Portugal saved both break points while defeating Dutchman Robin Haase 6-3, 6-2 in 57 minutes. Sousa was lifted by the support of the crowd.
"The crowd is great," said Sousa. "The centre court is one of the nicest of 250 (tournaments) I have played in. It's very comfortable to play and I feel very good."
Sousa now faces eighth-seeded Marcos Baghdatis of Cyprus, who had 12 aces while beating defending champion Jiri Vesely of the Czech Republic 6-2, 6-4.
After failing to win a single match in his last three appearances in this event, Baghdatis is now one victory from the finals.
"Now I play more with experience, more with the mind," said the 31-year-old Baghdatis. "When you are young you just go out there and hit all the balls and (with) no fear so you don't think so much. Nowadays you have to think a bit more, try to find different ways of confusing the opponent and I'm doing that right now."