John Isner has admitted to feeling mixed emotions after beating fellow American Marcos Giron in the first round of the US Open on Tuesday night.

Giron, the reigning NCAA champion, battled impressively throughout the straight-sets defeat to Isner, who concedes that it is always difficult to play an opponent from America at the tournament.

"That first set really could have gone either way," the 13th seed told reporters. "I thought Marcos played very well, but I thought I played well today, too. I knew it was going to be a tough match going in.

"It's tough playing another American, especially at the US Open, because one of us is going to go home.

"It's going to happen a lot in the future when I'm playing guys that are younger than me that are American. It's a lot of pressure for me, I guess, because I'm expected to win that match. But at the same time it's good. It's good competition."

Isner will meet Jan-Lennard Struff in the second round.