Alexander Zverev admitted to being surprised by the defensive qualities of Darian King after being forced to work hard to beat the Bajan in round one of the US Open.
Fourth seed Zverev, one of the favourites for the men's singles title, needed the best part of three hours to see off the world number 168 at Flushing Meadows.
King was eventually beaten 7-6 (11-9) 7-5 6-4, having impressed his more illustrious opponent.
"He surprised me a lot with how well he handled my fast balls," said Zverev. "I obviously didn't play my best, but I thought I was hitting the ball well. He always had answers to my quick balls, to my pace. He's a very, very quick player.
"So I'm happy to be through. Obviously the ranking says he's, whatever, 150 in the world. He definitely didn't play like it today.
"I knew who he was, of course. But not about his game. So he surprised me a lot, how good he was defensively and how well he could handle the fast-paced balls."
Zverev, who will face Borna Coric in round two, added: "Obviously you can't be satisfied after a first-round win. I obviously want to go deeper.
"I'm going to be on the practice courts tomorrow trying to improve."