Roger Federer dug deep before winning in five sets ©Getty Images

Five-time champion Roger Federer was taken to five sets for the second time in three days before winning again against Mikhail Youzhny of Russia at the US Open in New York.

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).

Swiss fans cheer on Roger Federer during his five-set victory ©Getty Images
Swiss fans cheer on Roger Federer during his five-set victory ©Getty Images

"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.