Novak Djokovic needed extensive treatment on a shoulder injury during his second-round win over Juan Ignacio Londero at the US Open.

The defending champion and world number one came through a tough encounter in straight sets, 6-4 7-6 (3) 6-1, despite falling a double break behind in the second.

Ordinarily Djokovic is not a player who discloses any injury problems, but this one was difficult to hide and could hinder his title defence.

He admitted: "It was definitely affecting my serve and my backhand.

"I was definitely tested. This is something I have been carrying for quite a while now. It wasn't easy playing with the pain but you have to find a way.

"I had some luck in the second set. I'll move on, it's not the first time I have faced this challenge. It is what it is. Now I'll probably freeze my arm for 48 hours.

"It was a straight-sets win, but it was a difficult match to play. He was unfortunate at 3-0 up with serve in the second set, but somehow I managed to come back, that was crucial."

Roger Federer was at a loss to explain his sluggish starts this week after dropping the opening set for the second successive match.

Two days after Indian qualifier Sumit Nagal pinched the first set, Federer found himself behind again, this time to world number 99 Damir Dzumhur.

The five-time Flushing Meadows champion was broken in his first service game thanks to a wayward volley and a loose forehand into the net.

Dzumhur, from Bosnia, then secured an unlikely double break to take a 4-0 lead on his way to clinching the first set.

The drama almost turned into a crisis as Federer faced a break point in the second, but he survived a long, tense rally to come out unscathed.

From then on the 20-time grand slam champion took control, wrapping up a 3-6 6-2 6-3 6-4 win in two hours and 22 minutes.

But the error-strewn start to the match was a cause of some concern for the 38-year-old.

"I got exactly what I expected from both guys. I knew what Nagal was going to give me. I knew what Dzumhur was going to give me," he said.

"But I didn't expect to hit 15 to 20 unforced errors, which is basically the entire set just sort of donated.

"But look, they came out and they were well prepared and got me to do that. But I clearly have to play better from the get-go.

"When it happens like this back-to-back matches it's just a bit frustrating more than anything, especially when the level is that low and there is that many errors and the energy is not kind of there.

"But, yeah, I can only do better, which is a great thing moving forward."

Seventh seed Kei Nishikori, a finalist in New York in 2014, beat American Bradley Klahn 6-2 4-6 6-3 7-5 on a day otherwise wrecked by rain, which meant no play on any of the outside courts.