John Isner was just a pup (a really big pup) when he first faced Roger Federer. It was at the U.S. Open in 2007 and Isner was just out of college and just on the entrance ramp to a pro career.

By then Federer was a fully fledged Grand Slam winner and the No. 1 player in the world. But the towering Isner (6-10) served his way to a first-set win. The American crowd was ecstatic, even though Federer would win the next three sets handily to claim the third-round match. Monday night, Isner took on Federer in the round of 16 on Arthur Ashe Stadium court.

"That was eight years ago now," says Isner, the highest-seeded American in the Open at No. 13. "Absolutely no pressure on me that day, and I honestly probably didn't believe I could beat him, either. I was happy to be on the court. I was fresh out of college and no one knew anything about me. He certainly didn't. I won one set, which was incredible. Didn't win much after that."

While a winner of ATP Tour events, Isner is still looking for his first Grand Slam title. And for him the one match he has won over Federer in the five they have played is his biggest career win. That was a Davis Cup victory in 2012, coming back after dropping the first set to win in four.

"I think that was the best win of my career and one of the best matches I ever played hands down," Isner said.

Isner will ever be remembered for playing the longest match in the history of tennis at Wimbledon in 2010. It took him three days, 11 hours and five minutes, and a 70-68 fifth set to defeat Nicolas Mahut.

Isner's powerful serve is his primary weapon. In Federer, who owns not only a record 17 Grand Slam titles but the most complete game of all time, Isner faces a very good returner who has added a wrinkle to his game this year, rushing some second serves to chip them early and hurry his opponent into a fatal mistake.

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"He's one of the best," Isner said. "He has that chip return that no one else has. That's the shot that gets him points. From that, his talent and skill and everything, everything takes over.

"He's able to get a lot of balls back, and he uses his incredible skill and athleticism to win points after that. So he's a very, very good returner."

Federer has played only those five matches against Isner since they first met in 2007 and they haven't played in three years. Federer says he can't remember Isner's serve that well, but went on to describe its challenges.

"John, it's been once every two years maybe, so I don't know [his serve] that well," Federer said. "He's got the power. Then clearly because he's so tall he finds the impossible angles for us. And he's got a great second serve. Obviously, best-of-three-set match he's even more dangerous. Best-of-five you feel like you have a bit more time, but he clearly can also run three, four, five sets serving great."

Federer, at 33, is serving possibly the best of his career. So Monday night's match promised to serve up some of the Open's most entertaining tennis.