Roger Federer played down concerns over his back injury after a hard-fought five-set win over Frances Tiafoe in the US Open first round.

The Swiss great was pushed by American teenager Tiafoe on Tuesday, battling to a 4-6 6-2 6-1 1-6 6-4 victory on Arthur Ashe.

Federer made a slow start, dropping a set in the opening round at Flushing Meadows for the first time since 2003.

And, for just the fourth time in his illustrious career, the 19-time grand slam champion was pushed to five sets in the first round of a major.

But Federer, who blamed his early struggles on not being able to prepare as he would have liked, dismissed suggestions he was having issues with the back injury that troubled him heading to New York.

"I'm very happy that I'm feeling as good as I'm feeling right now," he said. "Honestly, if I would have felt like that going into this tournament that my back was going to get worse every match, I probably wouldn't have played.

"My hope and my belief is that it's only going to get better from here because every day that goes by puts me further away from what happened in Montreal. So that's how I see it.

"I'm very happy how I'm feeling right now, how I felt coming off the court. I think there's a lot of positives for me to look at, that I can come through a five-setter with the preparation that I had.

"I was definitely also lucky to play tonight because the rain. Others got all pushed over to tomorrow [Wednesday]. So that would have been tougher for me maybe to back it up back-to-back days.

"Look, if I enter the tournament, it's because I believe I can play and go deep. I still believe that.

"I think this will actually give me a lot of confidence. In the first set really I was just seeing and feeling it, see how far I could push, but I was never in pain. It was OK.

"That's why when I was down a set, I just said, 'OK, the match starts here. One set all, OK, it's a best-of-three set match.' Being in the fifth set, I said, 'It's great I'm still in the match. Things are actually great.' I always try to keep a very positive mindset out there."

Federer will face either Blaz Kavcic or Mikhail Youzhny in the second round, with the Swiss star and Rafael Nadal the only men able to complete their opening matches on a rain-affected day.