American wildcard Frances Tiafoe has eliminated No.2 seed Juan Martin del Potro in three tight sets in the second round of the Delray Beach Open in Florida.
Tiafoe saved 14 of 16 break points against him on Thursday night to edge the Argentine 7-6 (8-6) 4-6 7-5.
Tiafoe hit 17 aces and won a three-setter for the fourth time this month.
No.8 Chung Hyeon became the highest-seeded player to reach the quarter-finals when he eliminated Franko Skugor 6-4 7-6 (7-4).
American Jared Donaldson beat Denis Shapovalov of Canada, 7-6 (8-6) 4-6 4-6.