American qualifier Christian Harrison upset top seed Cristian Garin of Chile in the second round of the Delray Beach Open.
Harrison, ranked No.789, prevailed 7-6 (7-3) 6-2 to double his career wins on the ATP Tour to four and reach his maiden top-tier quarter-final.
The 26-year-old - whose career has been hampered by multiple surgeries - squandered three consecutive set points serving at 6-5 in the first set and lost serve, but regrouped to dominate the tiebreaker.
He swept the final four games of the match to become the second-lowest-ranked quarter-finalist in tournament history.
"I just love the sport, I love what I'm doing," Harrison said.
"I just can't wait to keep playing and hopefully have a good, healthy next couple of years."
Former world No.3 Juan Martin del Potro was ranked No.1042 when he reached the quarters in 2016, on his caomeback from injury.
Garin, who had a first-round bye, had no answer to Harrison in the cool afternoon playing conditions.
"These conditions are good for him," Garin said.
"He played really flat, really fast. I hope he can play like this the rest of the year."
Harrison next plays Gianluca Mager of Italy, who advanced by beating sixth-seeded and 2016 champion Sam Querrey 7-6 (8-6) 6-1.
Second-seeded John Isner hit 22 aces and never faced a break point in his 6-4 7-6 (7-4) win over Brazil's Thiago Monteiro.
The big-serving American will next face Sebastian Korda, who rallied to win an all-American match against fifth-seeded Tommy Paul 4-6 6-3 6-2.\ep playing and hopefully have a good, healthy next couple of years."