John Isner feels as though he can come up with an overpowering serve whenever he needs it. He showed it on Friday.

Isner, the top seed and defending champion, advanced to the semifinals at the Hall of Fame Tennis Championships by beating South Africa's Izak Van Der Merwe 6-4, 7-6 (2).

The 6-foot-9 Isner will face No. 6 seed and fellow American Ryan Harrison, who also advanced to the semis Friday on Newport's grass courts. Harrison was ahead 6-4, 3-0 when Germany's Benjamin Becker retired with a left hamstring injury.

Isner and Harrison will meet on center court Saturday following Tennis Hall of Fame induction ceremonies. Jennifer Capriati and Gustavo Kuerten headline the 2012 class.

Lleyton Hewitt of Australia and 2009 Newport champion Rajeev Ram will meet in the other semifinal.

Isner broke in the third game of the first set, going up 3-1, and closed out the final game of the set at love.

In the second set, Isner trailed 5-6 and was in danger of being broken in the 12th game. He trailed love-30, but came back with a hard serve down the middle for an ace. He closed the game by winning four of the final five points.

"More times than not, when I'm in a match and when I really need a big serve, I tend to come up with it," he said. "At the very least I hit my fastest serve because I tend to have the adrenaline. It's something I've said a million times — it's a weapon I'm lucky to have and it's sort of a get out of jail free card."

Isner then broke on the first point of the tiebreak. His 12th and final ace of the match gave him a 5-2 edge.

"It's huge. It happened yesterday," he said of the quick start in the tiebreak. "I always say when I go up 1-0 on a break in a tiebreak, there's a good chance it's going to be 3-0."

Isner improved to 23-10 in tiebreaks, best on the ATP tour.

He's is looking to become the first Newport champion to successfully defend his title since Greg Rusedski in 2005.

In the second quarterfinal match, Harrison overcame four break points in the final game of the first set. He's saved 19 of 22 break points this week.

"It's big. Everyone's going to have that point in the match where you get hot and win the big points," he said.

During the break, a trainer worked on Becker's left leg and back. He was broken in the first and third games of the final set before retiring.

Before Harrison and Becker took the court, Isner was asked to compare the two he could have faced.

"Ryan thinks he can beat anyone in the world," he said, smiling, before saying he thought they were fairly even.

Harrison smiled when he was told what Isner said.

"I think John has that same attitude, too," he said. "Anybody that's played at a high level of any sport has to have that attitude."

Hewitt and Ram both advanced in straight sets on Thursday.