Roger Federer kept his unblemished second-round record at the US Open intact after he came from a set down against Bosnia and Herzegovina's Damir Dzumhur to win 3-6 6-2 6-3 6-4.
Swiss third-seed Federer, who overcame a first-round scare from India's Sumit Nagal before prevailing in four sets on Monday, was hoping for an easier outing against Dzumhur under the roof of the Arthur Ashe Stadium.
But a lethargic Federer received a shock when the world No 99 took the opening set. The five-time US Open champion woke up and hit back as he won the next three sets to extend his perfect record to 19-0 in second-round matches at Flushing Meadows.
Federer struck 58 winners, including 16 aces, but he also made an uncharacteristically high 45 unforced errors as he advanced to the third round where he could meet British No 2 Dan Evans.
Japan's Kei Nishikori fended off Bradley Klahn 6-2 4-6 6-3 7-5 as the 2014 Flushing Meadows runner-up continued his bid for a first Grand Slam title.
Nishikori, who had rued his lack of playing time after his first-round opponent was forced to retire, was joined by his usual crowd of excitable fans, shielded from the soggy conditions under the Louis Armstrong Stadium's retractable roof.
He will face either 31st-seed Cristian Garin of Chile or Australia's Alex de Minaur in the third round.