Fifth seed Alexander Zverev was taken to four sets by American teenager Brandon Nakashima in a hard-fought victory in the US Open second round.
The German, one of those tipped to challenge for the title in New York in the absence of Rafael Nadal and Roger Federer, won 7-5 6-7 (8-10) 6-3 6-1.
Zverev, whose serve was unbroken in the match, hit 24 aces past the 19-year-old.
He will face French 32nd seed Adrian Mannarino or American Jack Sock next.