The fourth-seeded Czech advanced to Sunday's final and will face Mardy Fish, who cruised to a 6-3, 6-2 victory against injured doubles partner James Blake in the other semifinal.
Roddick dominated the first set, banging 10 aces. But the momentum swung in the second set, as Stepanek countered Roddick's power with consistency, taking some pace off his shots.
"Definitely it feels great,'' said Stepanek, who lost to Roddick in straight sets in the SAP final last year and beat him Saturday for the first time in five tries. "He's a great player. He has an unbelievable record here. ... I had a little revenge from last year in the finals.''
As the match wore on and Roddick's mistakes mounted, he became more and more frustrated. After losing a point in the second-set tiebreaker, he slammed his racket to the ground. In the third set, he threw his racket two more times and launched a tennis ball into the upper reaches of HP Pavilion, home of the San Jose Sharks.
"I played all right,'' Roddick said. "I got frustrated because I wasn't able to bear down and execute. He played well. As badly as I thought I executed, he did a pretty good job. He just does the basics so well. He puts first serves in. He puts the ball in the corner. He puts the ball where it's supposed to be, comes in on the right shots and kind of stays there mentally, also. It's a credit to him.''
Roddick said the only thing he's "upset'' about is that he got so "mad'' during the match.
"I didn't just stay the course,'' Roddick said. "Because I have been doing that real well this year. So there's definitely something to build on next week.''
After the match, Stepanek celebrated with his trademark "worm dance'' and put on a Sharks jersey, playing to the crowd.
Stepanek, ranked 21st in the world, is 2-0 all-time against Fish, beating him twice in 2007, both times in straight sets. Stepanek has three career victories, including one earlier this year at Brisbane.
"Stepanek obviously plays well here finals last year,'' Fish said. "He presents a lot of problems. He serves well. He returns well similar style of play to myself. Moves extremely well. One of those guys who literally will do anything to win.''
Stepanek showed his comeback ability in a quarterfinals victory Friday over Todd Widom. He dropped the first set then stormed back to win the next two, 6-0, 6-0. Then he pulled off a much bigger comeback against the world's No. 6 ranked player and a three-time winner of the SAP Open.
Fish, the tournament's fifth seed, reached the final in San Jose for the second time. In 2004, he beat Andre Agassi in the semifinals but lost to Roddick in the title match, 6-7 (13) 4-6.
Blake, the tournament's second seed, rolled his right ankle late in the final set of his victory Friday night over Sam Querrey. It quickly became apparent Saturday, as Blake limped after shots, that he was still in pain.
"I had no idea his ankle was that bad,'' Fish said. "It's just a testament to James. He's never defaulted in a match in his career. He stuck it out.''
Trailing Fish 1-2 in the first set and limping badly, Blake called for the trainer, who retaped his right ankle. Blake called for the trainer again at the end of the first set and had his ankle retaped for a second time.
"It affected me more than I thought it would have when I did it (Friday),'' Blake said. "I just iced it a few times yesterday and taped it up. I thought I'd be ready to go. I wasn't exactly ready.
"I know it's tough for Mardy to play against someone like that, especially a friend, but he did a good job. ... Obviously he deserves to be in the finals. This is probably the only time in my career I can say I'm actually happy someone else beat me. I wouldn't have been able to perform tomorrow. I wouldn't have been able to be out there deserving of being in the finals. The right person won today. I'm happy for him.''
Fish said it was during Blake's first service game that he realized how badly he was hurt.
"You just have to kind of forget you're playing one of your best friends,'' Fish said.
Blake had particular trouble against Fish's serve. He was aced 16 times, often barely moving as shots zoomed past.
Blake said his trainer assured him he wouldn't risk a more serious injury by playing. So he played on.
"I thought if I wasn't going to hurt it any more, I could suck up the pain for 20 minutes, 30 minutes and make it a little better for the fans, hopefully, and better for Mardy to feel like he earned a victory, which he did,'' Blake said. "He did a great job of staying focused the whole time, especially against a friend that's injured.''
Blake and Fish withdrew from their semifinals doubles match against Stepanek and Tommy Haas, that was scheduled for late Saturday night.
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Stepanek rallies to win SAP Open
Stepanek rallies to win SAP Open
