Fifth-ranked Roddick fired 12 aces and a crucial backhand winner on the penultimate point to defeat big-serving Croatian Ivo Karlovic 7-6 (9/7), 7-6 (7/5), giving the US star a chance to play for his 23rd career title on Sunday.

Isner, who made his ATP debut a month ago, continued his astonishing streak of winning every match this week in a third-set tie-breaker, doing so for the fifth time to oust Frenchman Gael Monfils 6-7 (4/7), 7-6 (7/1), 7-6 (7/2).

Roddick, who spoke with Isner this week for the first time, will meet the 22-year-old in the first all-US ATP final since Roddick played James Blake last year at Indianapolis.

In only his second ATP event, 6-foot-9 Isner has made himself the flashy new hope for US tennis, firing 132 aces this week, including 23 against Monfils with a 74 percent first-serve accuracy rate.

"Maybe he is the best server on the tour now, with Karlovic," Monfils said. "I've never played a guy who serves like this. With his serve, he can be in the top 10 easy."

Tall ATP aces leader Karlovic, who added 19 against Roddick to his 700 earlier this season, suffered his first loss in four semi-finals this year. His style was perfect preparation for Roddick's first taste of Isner's hot serve.

Isner, 22, was not even supposed to play this week, only claiming a wild card when sixth-ranked Fernando Gonzalez of Chile pulled out late last week.

Roddick, the 2001 and 2005 Washington winner, scrambled to his left and swatted a passing backhand winner past seventh seed Karlovic to grab a 6-5 lead in the last tie-breaker. Karlovic netted a backhand to fall in 97 minutes.

Roddick, who turns 25 later this month, reached his third ATP final of the year, having won at Queen's in June but lost at Memphis in February.

Isner mastered last-set pressure to eliminate Britain's Tim Henman, German second seed Tommy Haas, German eighth seed Benjamin Becker, American Wayne Odesnik and saved the greatest dramatics for eliminating speedy Monfils.

Monfils took the first break in either semi-final to grab a 6-5 lead in the third set, but gave the break back with two wide forehands to fall into Isner's tie-break zone.

Monfils hit an ace for a 2-1 lead and never won another point, making two unforced errors followed by four Isner winners.

"I'm not arrogant, but I was so much better than him," Monfils said. "I'm so pissed. It's tough when you are so much better than another guy and then he beats you."