Federer's never-before-seen low in 'worst 15 minutes of tennis'

Roger Federer looked up at the Arthur Ashe Stadium scoreboard and for the second time in three days saw it lit up with a strange sight.

It showed Federer down a set to a player ranked far below him.

The five-times champion came back again, beating Damir Dzhumhur 3-6 6-2 6-3 6-4 on Wednesday to reach the third round of the year's final grand slam.

With rain affecting play in Flushing Meadows for the first time in the tournament, only the matches on Arthur Ashe Stadium and Louis Armstrong Stadium, which have retractable roofs, were able to be played in the early afternoon.

The covered conditions were no help at the start to Federer, though he couldn't quite explain why.

"I don't have an answer to you," he said. "It's just poor ball striking in the beginning."

Before the tournament, Federer had never dropped the opening set in his first two matches at a grand slam event.

At Flushing Meadows on Thursday, the Swiss made unwanted history.

Respected tennis journalist Jose Morgado described Federer's poor start as "possibly the worst 15 minutes of tennis" he's seen the Swiss great play.

In more bad omens for Federer, no player has dropped the opening set in his first two matches and won the US Open title since the challenge round was abolished in 1912, according to the ATP.

Third-seeded Federer had a night match on Monday and dropped the first set to qualifier Sumit Nagal before he won in four sets.

It was another slow start against Dzumhur, whose No.99 ranking was just good enough for direct entry.

Federer, winner of a men's record 20 major singles titles, was a little frustrated but not entirely surprised.

"I mean, look, I got exactly what I expected from both guys," Federer said.

"I knew what Nagal was going to give me. I knew what Dzumhur was going to give me.

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

Next up for Federer will be French 25th-seed Lucas Pouille or Briton Dan Evans.

Kei Nishikori relied on his strong baseline retrieving game against Bradley Klahn to advance 6-2 4-6 6-3 7-5.

The Japanese seventh-seed struggled at times on his first serve and needed five match points to close it out.

"A little bit of lost focus," Nishikori said after the match.

Off the court, world No.12 Borna Coric pulled out the tournament with a lower back strain, giving a second-round walkover to Grigor Dimitrov.

With AAP