Defending champion Novak Djokovic battled through a shoulder injury to make the US Open third round with a win over Argentina's Juan Ignacio Londero.
The top seed needed treatment three times, but still went on to win 6-4 7-6 (7-3) 6-1.
"It was definitely affecting my serve and my backhand," said Djokovic, who lost his serve five times.
The Serb added he had not felt that "kind of sensation too many times" in his career.
"I didn't know if I would be able to finish the match. I'm really glad I have," he said.
Fellow Serb Dusan Lajovic will be Djokovic's next opponent if he beats American Denis Kudla.