US Open: Novak Djokovic overcomes shoulder complaint to book place in third round alongside Roger Federer

Djokovic received treatment for shoulder injury he has been carrying 'for quite a while now' before seeing off Londero 6-4, 7-6, 6-1

Reigning champion Novak Djokovic battled a shoulder problem that hampered his serve and backhand as he secured a 6-4, 7-6, 6-1 win over Argentine Juan Ignacio Londero and reached the third round of the US Open.

Djokovic, who had work done on his left shoulder during his pre-match warm-up inside Arthur Ashe Stadium earlier in the day, was in clear distress when he took a medical timeout while leading 4-3 in the first set.

"I was definitely tested and this is something I have been carrying for quite a while now," Djokovic said when asked about the injury.

"It wasn't easy, obviously, playing with the pain but you have to find a way to fight and hope you get some lucky shots."

Djokovic returned to court to close out the first set and then had more treatment before Londero, playing in only his second main draw match at Flushing Meadows, broke him twice for a 3-0 lead in the second.

So bothered was Djokovic by his backhand that in the third game of the second set, at 30-30 and with Londero well out of position, he failed to get even a soft backhand to the open court over the net.

But he roared back to take the next five games, a stretch during which he managed to fire off a number of backhand winners, en route to strolling through the tiebreak before getting more treatment on his shoulder.

While Djokovic was nowhere near his best, Londero, who is in the midst of a breakout season that saw him win his first title, will be happy to have played at such a high level in his first career meeting with the Serbian.

Djokovic won't play again until Friday and until then, "I'll probably freeze my arm for 48 hours, not do anything with it and see what happens."

Up next for top seed, who has won four of the last five grand slams, will be either fellow Serbian Dusan Lajovic, the 27th seed, or American Denis Kudla, who were unable to play their match earlier because of inclement weather.

Roger Federer shook off another slow start to advance to the third round.

Federer, a 20-time grand slam winner seeking his first US Open crown since 2008, rallied to beat 99th-ranked Bosnian Damir Dzumhur 3-6, 6-2, 6-3, 6-4.

The 38-year-old Swiss third seed, who also dropped the first set against Indian qualifier Sumit Nagal in his opener, answered the wake-up call after conceding the first set with 17 unforced errors.

"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," Federer said. "But can only do better, which is a great thing moving forward.

"I didn't expect to hit 15 to 20 unforced errors, which is basically the entire set just sort of donated ... I clearly have to play better from the get-go."

Only 10 of 32 singles matches were played Wednesday as rain wiped out play except in enclosed stadiums, with Russian fifth seed Daniil Medvedev and three-time grand slam champion Stan Wawrinka among those postponed to Thursday.

"I definitely profit from everything I did in the game and my ranking to be put on center court on a day like this," Federer said.