* Swiss showman leaves them breathless in the Big Apple
* Hopes to keep mouthwatering quarter-finals date with Nadal (Adds quotes, details)
By Larry Fine
NEW YORK, Aug 31 (Reuters) - Grand slam king Roger Federer showed off his fitness by sprinting into the round of 16 at the U.S. Open on Saturday, beating Adrian Mannarino 6-3 6-0 6-2 to turn out the lights on Day Six of the tournament.
Playing the closing contest of Saturday's night programme, Federer ran through games as though he had a taxi cab waiting outside the stadium with the meter running and hurtled past the unseeded Frenchman in 81 minutes.
"I like it like this," said Federer. "I like three hours, five hours, too, as long as I end up winning them. But no doubt about it, you do feel more of the show factor at night than during the day.
"But it does feel great just for confidence, and I had a lot of fun out there."
The one-sided victory moved Federer, the greatest grand slam men's title winner of them all with 17, into a fourth-round clash with Spain's Tommy Robredo, who earlier beat British qualifier Dan Evans.
The 32-year-old Swiss has won all 10 of his previous matches against Robredo.
Federer might not want to get ahead of himself, no matter how quickly he moves past opponents, but it was hard not to notice that he was now one step away from a potential quarter-final showdown with rival Rafa Nadal.
Strange as it may seem for rivals who have squared off 31 times over their careers, five-times U.S. Open champion Federer has never played 2010 winner Nadal at Flushing Meadows.
"Clearly (I) follow the progress of the players, but today I didn't see anything during the day," Federer said.
"I think I only start really focusing on it once I'm really right there, like the moment I would win my next round and he did the same.
"(But) I have gone through that my entire career, people talking about our matches even before the tournament started," he added.
"Clearly I think we both hope it's going to happen this time for the first time in New York."
Federer said he had several things working in his favour against Mannarino on Saturday.
"I was able to really use my serve well, because it was breezy tonight again so I was able to use the wind a bit better. Maybe I had a bit more variation than him that allowed me more margin in my game.
"Once I had the first set I was able to play with the lead, which makes things a little more easy as well."
It got so easy for Federer that he yielded just 11 points to the Frenchman in the second set. In the match, the Swiss slashed 34 winners to eight for his 25-year-old opponent.
Federer showed absolutely no signs of the back issues that have limited him at times in 2013, a year that has seen him win just one tournament title and his U.S. Open seeding slip to seventh, his lowest mark in a decade.
A second-round elimination at Wimbledon led some to question whether the Swiss master was slowing down.
The masterclass he gave on centre court of the National Tennis Center seemed to allay those concerns.
At one stage, Federer was playing at such a pace the local broadcaster was unable to get through their commercials before the players were back on court and fans watching on television rejoined the match in the middle of the next game.
Federer said he still got a charge out of the big-night atmosphere under the lights in Arthur Ashe.
"In this stadium, with this crowd, it's always very particular, clearly, because it is the biggest stadium in the world, it is New York City, and you don't ever know how many times more you're going to play on this court.
"You always want to enjoy it."
Reporting by Larry Fine, Editing by Nick Mulvenney