HOUSTON -- John Isner apologized for his performance. Dustin Brown said it felt unbelievable.

The top-seeded Isner lost to the dreadlocked Brown in the second round of the U.S. Men's Clay Court Championship on Wednesday night.

It was Brown's first career victory against a top-10 player. The German closed out a 6-4, 6-7 (7), 7-6 (4) win with a forehand winner.

"I've had a lot of matches where I've played good tennis, where I was close, and you've just got to keep playing," the 29-year-old Brown said. "I've got to play my tennis. If I start playing the tennis (his opponent) wants, that's not good for me.

"He's been in these situations much more often than me -- I got a little tight in the second set, maybe at the end of the third -- so I had to try to keep playing. It feels unbelievable."

The 6-foot-5 Brown, who dropped each of his previous three matches against opponents ranked 10 or higher, has spent most of his career on the Challenger circuit, where he has won five titles.

Isner, ranked No. 9, was the defending champion in Houston. He had 13 aces in the loss.

The 6-9 Isner apologized following his post-match interview to a tournament official for a "pathetic performance -- I'm sorry you had to see that."

"I just didn't feel it tonight," he said.

American Sam Querrey also advanced with a 6-3, 6-4 victory over No. 7 seed Lleyton Hewitt. Querrey has been struggling, dropping to 82nd in the ATP rankings.

Sixth-seeded Juan Monaco beat Somdev Devvarman of India 7-6 (4), 6-2 for his 200th career win on clay.

Also advancing was Santiago Giraldo of Colombia. He made short work of Matthew Ebden 6-1, 6-2 to set up a second-round meeting with No. 2 seed, Tommy Robredo, of Spain.