JOHNS CREEK, Ga. — Andy Roddick was surprised by the power and accuracy of Xavier Malisse's serve.

Needing to give himself a chance to break serve late in the third set, Roddick dropped about 8 feet behind the baseline to show the Belgian a different look. The strategy paid off.

"I wasn't returning well close in," Roddick said. "I just didn't have my timing or whatever. I think he missed eight of his first nine serves when I stood back and did that."

Roddick, the No. 1 seed, beat Malisse 4-6, 6-3, 6-4 in the quarterfinals of the Atlanta Tennis Championships on Friday night.

John Isner, the second seed, took less than an hour in the afternoon to beat Michael Russell 6-1, 6-2.

Roddick pumped his fist and shouted in celebration after breaking serve on Malisse's wide backhander.

"He was in a pretty decent groove on first serves in the third set. He was hitting his spots. He has a good serve, but he doesn't normally pop aces, you know? I gave him a different look."

Before improving to 9-0 on the ATP tour against Malisse, Roddick struggled in the first set with two double-faults and had problems throughout the match in earning just six of 38 first-return points.

But Roddick, the tour leader this year with a 28-4 record on hard courts, never lost the power of his serve and finished with 17 aces. The capacity crowd at the Atlanta Athletic Club's stadium court was decidedly in his favor.

Malisse, the seventh seed, wasn't upset that many fans cheered after he hit errant shots.

"It's nice to have a crowd, at least," Malisse said. "Even if it's against me, it doesn't really bother me.."

Isner, who rose to tennis fame with a record-long match at Wimbledon, showed why he's one of the ATP tour's service leaders with 11 aces and an 89 percentage on first serves.

He will face South Africa's Kevin Anderson, a 6-3, 6-4 winner over Lukas Lacko of Slovakia, in the first semifinal.

Roddick, who played despite a swollen foot, will play Mardy Fish, a 6-3, 6-0 winner over Taylor Dent. Fish, who overcame a sprained ankle, is attempting to earn his second straight title after winning on grass two weeks ago in Newport, R.I.