Frances Tiafoe turned the tables on Juan Martin del Potro on Thursday, toppling the second-seeded former champion 7-6 (8/6), 4-6, 7-5 to reach the quarter-finals of the ATP tournament at Delray Beach, Florida.
The 20-year-old American needed four match points to finish off Argentina's del Potro, the latest seed to fall in a tournament that saw top seed and defending champion Jack Sock, sixth-seeded John Isner and ninth-seeded Milos Raonic beaten on Wednesday.
Tiafoe booked a quarter-final meeting with eighth-seeded South Korean Chung Hyeon, a 6-4, 7-6 (7/4) winner over tenacious Croatian qualifier Franko Skugor.
Del Potro, the 2011 Delray Beach champion and a semi-finalist the past two years, beat Tiafoe in straight sets in their most recent meeting in the first round of the Australian Open in January.
That followed a three-set victory over the young American, ranked 91st in the world, in Acapulco last year.
World number 10 del Potro appeared to be on his way when he seized an early 3-0 lead, but he was unable to capitalize on four set points as he dropped the opener in the tiebreaker.
The Argentinian opened the second set with a break and went on to level the match.
The tight third set saw del Potro save a match point against in the 10th game, and two more in the 12th before Tiafoe gained just the second win against a top-10 player in his career.
"It's unbelievable, definitely," said Tiafoe, who produced 17 aces and saved 14 of 16 break points he faced against the 2009 US Open champion."Two weeks in a row in the quarters.
"I really wanted that win to make a statement. I've had a ton of tough losses against top guys. Just wanted to get over the line."
In the quarters he will face Australian Open semi-finalist Chung, ranked 30th in the world, for the first time.
The 21-year-old South Korean had to dig in to finish off the 303rd-ranked Skugor, who rallied from 0-4 down in the second set, saving three match points as he forced the tiebreaker.
Chung prevailed on his fourth match point, when Skugor coughed up a double-fault.
Canadian 18-year-old Denis Shapovalov reached his first quarter-final of the season with a 6-7 (6/8), 6-4, 6-4 victory over 21-year-old American Jared Donaldson.
"I definitely picked up my serve in the second and third set," Shapovalov said. "It was a really even match. We were both playing extremely well."
Shapovalov next faces American Taylor Fritz, who fired 13 aces in a 6-1, 4-6, 6-3 victory over Russian veteran Mikhail Youzhny.
"There were a lot of times in the match where I was down and he had break points, and it could have really gone either way, and I came up in those moments in the match and that really made the difference," Fritz said.