A confident Roger Federer is delighted to have put injury concerns behind him as he chases a sixth US Open title at Flushing Meadows.

After being troubled by a back problem in the lead-up to the year's final grand slam, Federer was unconvincing in five-set victories over Frances Tiafoe and Mikhail Youzhny.

Yet the Swiss enjoyed a much more comfortable outing in round three, beating Feliciano Lopez 6-3 6-3 7-5 to remain on course for a semi-final meeting with great rival Rafael Nadal.

"I guess I maybe needed just a few hours here on this court," said Federer of his improved display on Saturday.

"I needed to get over the fear of the back issue, especially in that first match, the first set. Then I think the wobble I had in the second round was due to my preparation. I think now I'm just trusting my movement better.

"My serve is there. My mind is there. I can finally focus on playing tennis and not so much about the past. So it's nice to be in the present in my mind, looking ahead, thinking tactics.

"I'm happy where I am right now. I think we did the right thing by taking a slow approach in the preparation week. I think forcing it too early... I would have maybe had a setback and I couldn't have played at all. Who knows what would have happened.

"I think this way we rolled the dice a little bit. Now we are in the fourth round and things are looking much better. I'm really excited how I feel after two five-setters.

"I'm confident in my physical shape. I've done well in five-setters lately. I'm happy that today was straight. But I'm feeling better also."

Federer and Nadal are the clear favourites for the men's singles title in New York, with a host of seeded players having already exited a tournament missing injury victims Andy Murray, Novak Djokovic, Stan Wawrinka, Kei Nishikori and Milos Raonic.

While Federer is in line to meet Nadal in the last four, the bottom half of the draw has been totally shorn of star names, with Pablo Carreno Busta and Lucas Pouille now the highest seeds in that section.

"When you see a lot of seeds going out, I don't know, you naturally put the focus on you for it not to happen to you," said Federer.

"[There will be] a new Grand Slam finalist, I think, in the bottom section that we've never seen before, which is exciting in itself, I think.

"At the top we actually really have good players left. A lot of them can play at a very high level. They're all facing off now. I think it's an exciting tournament. I'm happy I'm still around.

"Rafa fought through well again. I'm happy for him, too. We'll see if it gets done or not. I'm curious to see myself."