Alexander Zverev withstood a stern challenge from qualifier Darian King to progress to the second round of the US Open in a match that finished past 2am in New York.
World number 168 King appeared in his first grand slam match but was not overawed and made the in-form Zverev battle to a 7-6 (11-9) 7-5 6-4 victory in the early hours at Flushing Meadows.
With Andy Murray, Novak Djokovic, Stan Wawrinka and Kei Nishikori absent from the year's final major, fourth seed Zverev has emerged as one of the players to beat at Flushing Meadows following titles in Washington and Montreal.
The 20-year-old racked up 62 errors but eventually showed why he is so highly rated, remaining composed to see off the tricky challenge of King and earn a second-round meeting with Borna Coric.
It was a sluggish start from Zverev, who was broken in the third game as King took a surprising 2-1 lead.
Showing it was not a fluke, King then saved two break points to consolidate his advantage before two passing shots earned him another chance to break, but Zverev this time survived.
That sparked Zverev - who has never progressed beyond the second round of the US Open - into action as he broke to put the contest back on serve with a stunning drop shot.
With momentum on his side, Zverev then went a break in front, only to hand it straight back to King and the youngster consolidated to level the match at 5-5 much to the annoyance of his opponent.
Zverev's frustrations increased as King refused to go way, forcing a tie-break, but the German recovered from 5-3 down and saved two set points to take out the first set.
The second set was a lot more straightforward, with neither player able to break serve through eight games.
Zverev eventually made the breakthrough in the 11th game as he moved 6-5 ahead and that was the opening he needed to take a commanding two-sets lead.
There was a sense of inevitability in the third set after Zverev claimed the early break and, although King refused to surrender, it was only a matter of time before the match was over.
Zverev  bt King 7-6 [11-9] 7-5 6-4
Zverev - 54/62
King - 23/42
Zverev - 6/5
King - 2/3
BREAK POINTS WON
Zverev - 4/11
King - 2/10
FIRST SERVE PERCENTAGE
Zverev - 57
King - 61
PERCENTAGE OF POINTS WON ON FIRST/SECOND SERVE
Zverev - 69/57
King - 64/48
Zverev - 138
King - 124