Roger Federer was back to his best last night as he saw off Spain's Feliciano Lopez in three straight sets to join Rafael Nadal in the US Open fourth round.

"The Maestro" extended his unbeaten record against the hard-serving Lopez to an astonishing 13-0.

The outcome of their clash is same as always, but everything else looked different, especially for the injury-bothered Swiss.

Federer, who came through five-set matches in the first two rounds, beat 35-year-old Lopez 6-3, 6-3, 7-5. 

Compared to Federer’s first two matches in US Open this year, the difference was clearly visible.

Against Frances Tiafoe and Mikhail Youzhny, Federer’s energy was low, which caused his serve to be attacked, and his backhand producing errors untypical of the Swiss.

None of those things were present against Lopez.

Federer conceded only one break, won 85 percent of his first-serve points, and unbelieveably out-aced his serve-bombing opponent, 13-9. The timing that was missing in round two had returned for the most part - with just four backhand errors.

Federer looked sharp, showing no sign of the back issue that marked his build-up to the tournament, and declared being positively surprised by the way he felt on the court.

"I'm really excited how I feel after two five-setters," the 19-time Grand Slam champion said.

"It was clearly nice to go up two sets to love for a change. Feels different from there.

"I needed to get over the fear of the back issue, especially in that first match, the first set," Federer explained. "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."

The Swiss star then went on to state he was surprised by how good he felt on the court.

"I had good energy because I think that was my biggest worry that after the two big five-setters I had I would feel slow or just maybe hard to throw the engine on or that I would have to force myself so much that I would get tired from that.

"But, it didn't happen and I got off to a good start."

Five-time champion Federer goes on to face Germany's Philipp Kohlschreiber in the last 16.

He and his long-time rival Rafael Nadal are two wins away from meeting each other in the semis. Interesting fact is that the two have never clashed at Flushing Meadows.

With lots of the top-seeders out of the tournament already, Federer's path to the trophy looks more clear.

If his back issues are behind him, it is hard to see someone stopping him from winning the US Open.