The 20-time Grand Slam champion struggled early, but battled back to dispatch 99th-ranked Dzumhur 3-6, 6-2, 6-3, 6-4 under the roof at Arthur Ashe Stadium. #Federer #USOpen

Roger Federer of Switzerland reaches to return a shot against Damir Dzumhur of Bosnia and Herzegovina during their Men's Singles second round match on day three of the 2019 US Open at the USTA Billie Jean King National Tennis Center on 28 August 2019 in the Flushing neighborhood of the Queens borough of New York City. (Katharine Lotze/Getty Images/AFP)

NEW YORK — Roger Federer shook off another slow start to defeat Bosnian Damir Dzumhur and reach the third round of the US Open on a rainy Wednesday when showers wiped out most matches.

The 20-time Grand Slam champion struggled early, but battled back to dispatch 99th-ranked Dzumhur 3-6, 6-2, 6-3, 6-4 under the roof at Arthur Ashe Stadium.

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 I 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.”

Five-time US Open winner Federer, who lost to top-ranked Novak Djokovic in last month’s epic five-set Wimbledon final, broke early in each of the last three sets and held serve to advance.

“I buckled down and told myself I was going to hang tough and not get broken and that made a big difference,” Federer said.

“I’m not concerned. I just need to take the positives out of it, because once I lose that first set I do get better, which is a good thing.”

Next in Federer’s path is the winner between French 25th seed Lucas Pouille and Britain’s 58th-ranked Dan Evans.

While play continued indoors at Ashe and Louis Armstrong, 22 second-round singles matches on outer courts were postponed by rain to Thursday, including matches involving Russian fifth seed Daniil Medvedev, coming off a title at Cincinnati, and three-time Grand Slam winner Stan Wawrinka.

Defending champion Novak Djokovic and six-time US Open winner Serena Williams were set for later matches under the roof at Ashe and French Open champion Ashleigh Barty, the second seed from Australia, was to play at Armstrong.

Djokovic, winner of four of the past five Slam titles and 16 in all, meets Argentina’s 56th-ranked Juan Ignacio Londero.

The Serbian star is trying to become the first back-to-back Open champion since Federer’s run from 2004-2008.

US eighth seed Williams, seeking a 24th career Grand Slam title to match the all-time record won by Margaret Court, faces 17-year-old US wildcard Caty McNally.

Williams had won her first Slam title at the 1999 US Open and reached her second Flushing Meadows final before 121st-ranked McNally was born.

Barty was set to play 73rd-ranked American Lauren Davis. The Aussie could meet Williams in the quarterfinals.

Japanese seventh seed Kei Nishikori, the 2014 US Open runner-up, eliminated 108th-ranked American Bradley Klahn 6-2, 4-6, 6-3, 7-5.

Nishikori, who could face Federer in the quarterfinals, stumbled after seizing a 5-1 edge in the final set.

Klahn saved a match point in the seventh game, another in the 10th before holding to level at 5-5 and two more in the final game before finally falling when he sent a backhand long.

“A little bit of lost focus after 5-1,” Nishikori said. “He started playing better too.”

Next in Nishikori’s path will be either Australian Alex de Minaur or Chilean 31st seed Cristian Garin.

Czech third seed Karolina Pliskova, seeking her first Grand Slam title, ousted 202nd-ranked Georgian qualifier Mariam Bolkvadze 6-1, 6-4.

“It was a solid performance,” Pliskova said. “I didn’t have to play the best today.”

Pliskova, the 2016 US Open runner-up, will next face either Tunisia’s Ons Jabeur or Aliaksandra Sasnovich of Belarus.

Pliskova must reach the last eight to have a chance at becoming world number one after the Open. Defending champion Naomi Osaka and reigning Wimbledon champion Simona Halep must win the title, while Barty has the inside track thanks to Osaka defending more points.