Roger Federer came from a set down to beat Damir Dzumhur 3-6 6-2 6-3 6-5 to reach the US Open third round.
Federer hit 16 aces to Dzumhur's two and 58 winners overall - more than double his opponent's 26.
"I don't think there is a secret to a good start other than warming up well, being well prepared mentally and not underestimating your opponent. I did all of that," said Federer, after a match played under the roof because of rain in New York on Wednesday.
"So when it happens in back-to-back matches it's a bit frustrating more than anything, especially when the level is that low and there are that many errors and the energy is not kind of there. But I'm not concerned because once I lose that first set I do get better, which is a good thing."
In the third round, Federer will face Dan Evans.