Miami - Wildcard Reilly Opelka, ranked 228th in the world, stunned top seed and defending champion Jack Sock 4-6, 7-5, 6-3 on Wednesday to reach the quarter-finals of the ATP tournament in Delray Beach, Florida.
Towering 20-year-old Opelka, playing in his first ATP level tournament of 2018, fired 17 aces past world number eight Sock to cap a day of upsets.
Opelka will next face Germany's Peter Gojowczyk, who sent sixth-seeded American John Isner packing 7-6 (7/3), 6-7 (4/7), 7-6 (7/5).
Unseeded American Steve Johnson dominated ninth-seeded Canadian Milos Raonic 6-2, 6-4.
Isner fired 30 aces and held three match points but couldn't hang on for the victory.
"I tried to go for my shots and not hold back," Gojowcyzk said. "He had three match points, but I played well. I think both of us were playing well."
For Isner it was another early exit. He accepted a wildcard into Delray Beach after falling in the first round of the New York Open last week.
"It was close. It happens," Isner said. "He made three straight first serves, and he hit a crazy, high, backhand drop shot, angle thing on the first match point, which was a good shot."
Johnson notched his first career win over Raonic in four meetings. It was a bitter departure for Raonic, who reached the Delray Beach final last year but had to pull out against Sock because of a leg injury.
Injuries limited Raonic's time on the court in 2017, and his first-round victory over Japan's Taro Daniel was his first Tour-level win in six months.
Ranked 51st in the world, Johnson made sure Raonic wouldn't build on that success. He withstood 11 aces from the Canadian - a former world number three now ranked 32nd - and surrendered only four points on his own first serve to book his first quarter-final appearance of the year.
"Any chance you get to play more matches that always builds confidence," said Johnson, who next faces Russian Evgeny Donskoy, a 7-6 (8/6), 6-3 winner over American Donald Young.
"The first set kind of got away from him with a second break so that is always comfortable," Johnson said.
Second-seeded Juan Martin del Potro will try to book his quarter-final berth on Thursday, taking on American Frances Tiafoe. Eighth-seeded Chung Hyeon of South Korea faces Croatian qualifier Franko Skugor for a place in the last eight.
Results from the ATP tournament at Delray Beach, Florida, on Wednesday (x denotes seeding):
Reilly Opelka (USA) bt Jack Sock (USA x1) 4-6, 7-5, 6-3
Peter Gojowczyk (GER) bt John Isner (USA x6) 7-6 (7/3), 6-7 (4/7), 7-6 (7/5)
Evgeny Donskoy (RUS) bt Donald Young (USA) 7-6 (8/6), 6-3
Steve Johnson (USA) bt Milos Raonic (CAN x9) 6-2, 6-4