Talented Croatian Borna Coric rallied from a set down to complete a stunning four-set comeback against fourth seed Alexander Zverev in the second round of the US Open.
Zverev emerged as one of the players to beat at Flushing Meadows following titles in Washington and Montreal and the withdrawals of Andy Murray, Novak Djokovic, Stan Wawrinka and Kei Nishikori.
But the in-form German was wasteful in New York on Wednesday, converting only one of 11 break-point chances after squandering the opportunity to win the fourth set at 0-40 in a 3-6 7-5 7-6 (7-1) 7-6 (7-4) loss.
Coric - through to the third round of the US Open for the first time in his career - will next face Kevin Anderson.
It was a case of who blinked first in the opening set as Zverev and Coric went head-to-head early in the battle of the 20-year-olds.
And Coric blinked, double-faulting at the worst time as he handed Zverev the break at 15-40 in the sixth game.
Zverev never looked back from that point, consolidating and closing out the set in a composed and comfortable manner.
It continued to be one-way traffic on Zverev's serve as Coric was unable to see a break-point opportunity through more than 40 minutes of play, though he finally earned a chance in the sixth game of the second set, which was immediately dismissed by the German's ace.
But as the set wore on, Coric had more looks at Zverev's serve and he brought up two set points.
Zverev dug himself out of a big hole with a stunning forehand winner down the line on set point, however Coric was not to be denied at the second time of asking to level the match.
That seemed to knock Zverev out of his stride, with the in-form player visibly frustrated as Coric grew in confidence.
Coric did not drop serve after being broken in the first set, though he was tested in the seventh and ninth games, fending off seven break points in the third alone.
And his grit and determination on serve saw him to a tie-break, which he won in a canter for a two-sets-to-one lead against a wasteful Zverev.
Zverev was even more frustrated at the start of the fourth set as an upset brewed, vocal and uncomfortable, and while it sparked him into action, he was unable to penetrate Coric's serve after wasting three set points.
Coric bt Zverev  3-6 7-5 7-6 (7-1) 7-6 (7-4)
Coric - 31/44
Zverev - 43/58
Coric - 5/3
Zverev - 22/7
BREAK POINTS WON
Coric - 1/7
Zverev - 1/11
FIRST SERVE PERCENTAGE
Coric - 68
Zverev - 67
PERCENTAGE OF POINTS WON ON FIRST/SECOND SERVE
Coric - 76/56
Zverev - 76/55
Coric - 151
Zverev - 148