It was another illustration of the Swiss superstar's remarkable fighting qualities as he seeks to add to his Australian Open and Wimbledon successes this year.

But the 36-year-old was clearly still struggling with the effects of a back injury at Flushing Meadows.

He powered through the first set against his 35-year-old opponent before losing the second and third. 

Edging the fourth proved key, however, as Federer then eased through the first to triumph 6-1, 6-7(3), 4-6, 6-4, 6-2.

It followed his five-set win over American teenager Frances Tiafoe on Tuesday (August 30).

"I think because you're on a high, you're thrilled that you got through, so you don't look at the negative," Federer reflected after being extended to five-sets in his first two matches at a Grand Slam for the first time.

"Or I don't. Yes, I might feel more tired than I normally would going into a third round, but that's okay.

"My preparation hasn't been good at all here. 

"I knew I was going to maybe struggle early on...maybe I struggled more than I would have liked to.

"But I'm still in the draw, which gives me a chance."

Seventh seed Grigor Dimitrov of Bulgaria was a major casualty today in the men's singles draw as he lost in straight sets to another Russian in 19-year-old Andrey Rublev.

Rublev won in three straight sets: 7-5, 7-6(3), 6-3.

Women's top seed Karolina Pliskova also had to recover from a set down in her second round match today.

The Czech player fought back to beat American Nicole Gibbs 2-6, 6-3 6-4.

Matches are still continuing today.