Federer owns a 6-2 lifetime record versus Monfils, most recently a three-set win at last month's U.S. Open tune-up event in Cincinnati, but is well aware of the Frenchman's ability.
"He's got easy top-10 potential," said Federer, who is bidding for his sixth US Open title and record 18th grand slam title overall.
"He's a great mover. He's got a wonderful serve, really, which nobody really talks about because of his athletic movement which stands out so much.
"I think I can speak on behalf of so many players: We love watching him play. It's nice seeing him do well again."
For all his undoubted talent, Monfils is into the quarter-finals of a grand slam for only the sixth time and his only semi-final came at the French Open in 2008.
Monfils, who has not dropped a set in his four matches here, described Federer as a "legend" of the game but the 20th seed believes he is playing well enough to cause the 33-year-old Swiss problems.
"Right now he's the greatest tennis player we ever had, and for me it's always challenging to play against him," said Monfils, who faces Federer at 8 p.m. ET (0000 GMT) on Thursday.
"I play tennis to play against a big legend, on a big court, show-time. That's what I train for. For me the best thing that can happen is to play against that guy on that stage."
Czech sixth seed Tomas Berdych takes on Marin Cilic, the 14th seed from Croatia, in the day's early quarter-final. (email@example.com)
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