This combination photo shows Roger Federer of Switzerland returning a winning shot between his legs while playing against Brian Dabul of Argentina during their opening-night match at the US Open in New York on Monday. Federer won 6-1, 6-4, 6-2 to advance to the second round and secure a place at the ATP World Finals in November. [Lucas Jackson / Reuters]

Roger Federer is becoming as deadly with his back to the net as he is when he faces up to his foes.

The world No 2 smacked another between-the-legs shot for the ages at the US Open on Monday in a 6-1, 6-4, 6-2 first-round victory over Argentina's 96th-ranked Brian Dabul in a night match at Arthur Ashe Stadium.

Federer's fantastic shot was similar to a jaw-dropping winner he hit against Novak Djokovic in last year's US Open semifinals, a between-the-legs blast for a crucial point as he was running from the net behind the baseline.

In the ninth game of Monday's second set, Federer hit a between-the-legs, back-to-the-net running forehand winner from behind the baseline, raising his arms and smiling after blasting the winner past Dabul, who stared and shrugged.

"Last year's was probably a little bit more important just because it was Love-30 to go Love-40, two points away from the match, and it was a semifinal, so obviously that has a little bit of an impact," Federer said.

"In terms of difficulty maybe this one was harder, because I had the feeling I had to run a longer distance and I was further back somehow. I had to really give the last big push at the end. I didn't have time to set it up."

The thrilling point, part of a 46-4 edge in winners for Federer, brought him to deuce and the crowd to its feet, another electrifying moment at a place where he has won five of his 16 Grand Slam titles.

"I felt like this one was incredible again," Federer said. "I turned around and couldn't believe the shot landed in the corner. The ovation was fantastic. Crowd went wild. You could see on my reaction that I couldn't believe it."

Federer never even saw the shot that lifted him to deuce, although Dabul would eventually hold his serve in the game.

"I had good contact, perfect speed and hit and everything," Federer said. "I hit it with it lower because then I can generate more pace off it. By the time I turned around it's already in the corner of the court on the other side."

The Swiss superstar held in the last game and went on the claim the match in 93 minutes, improving to 16-0 in US Open night matches and assuring him a spot in the ATP World Finals in November at London, joining top-ranked Rafael Nadal in having already qualified for the eight-man, season-ending showdown.

Federer said that while he will occasonally hit a between-the-legs shot in practice sesssions, such shots are not a regular part of his workout routine.

"I don't know if you can call it practice," Federer said. "I don't go like, 'Feed me a few of those.' It just happens or not. You probably hit one max at practice. So rarely."

Supreme confidence in shotmaking skills was not so much a part of the effort as a combination of necessity and luck, Federer said.

"There's not that much you can read into confidence when you hit a shot like that," Federer said. "You have to be at the net to have to hit a shot like that first, so it means I'm playing offensive, which is a good thing.

"My approach was somewhat awkward already to start with, so I don't know what I was thinking. I tried to fake him out and it didn't quite work."

The shot is destined to become a YouTube internet classic, much the way his video purporting to knock a can off a distant man's head with a served ball has become, although most of Federer's rivals think that was an arranged trick.

"They don't dare to try it," Federer said. "I can't tell you if it's real or not. I'm not going to answer that question.

"One thing I will tell you, the shot on center court in front of 22,000 people is a bit more difficult than what I did at the commercial. That was just having a bit more fun."

So could Federer smack a ball between his legs and knock a can off the head of someone standings behind him? Well, even a magician has his limits.

"That's kind of tough, the trajectory going up," Federer said. "You can feel the nose and everything being hit first. No, I wouldn't be able to do that."

Federer's on-court efforts have no doubters, however, but even he has not been able to assemble a list of his greatest shots in a decade-long career.

"I've pulled up some incredible shots throughout my career obviously in terms of length, just sheer talent or sheer importance of the moment because it all kind of comes together," he said. "Today was just great shot-making."