KEY BISCAYNE, Fla. — Andy Roddick whacked one last thunderous serve to finally beat Roger Federer, then slowly walked to the net with his head down, as if unsure how to celebrate such a rare achievement.

With an almost flawless performance, Roddick ended a streak of 11 consecutive losses against his nemesis today at the Sony Ericsson Open. Roddick dominated with his serve and took advantage of Federer's suddenly shaky play down the stretch to win 7-6 (4), 4-6, 6-3 in the quarterfinals.

"I came in knowing that nobody has beaten me 12 times in a row," Roddick told the delighted capacity crowd afterward. "So I had that on my side."

In the women's semifinals, Serena Williams overcame a slow start to outlast Svetlana Kuznetsova 3-6, 7-5, 6-3. Williams needs one more win for her fifth Key Biscayne title and second in a row.

"I haven't won anything five times, except I win all the time in Uno," Williams said. "I don't think that counts."

Her opponent Saturday will be No. 4-seeded Jelena Jankovic, who beat No. 19 Vera Zvonareva 6-1, 6-4.

Roddick improved to 2-15 against Federer, who came into the tournament hoping to quell talk that his game is in decline. Slowed by mononucleosis early in the year, the top-ranked Federer has yet to reach a final in 2008, making this his worst start since 2000.

"I am just sort of disappointed not to see my name playing in the finals," Federer said. "I think I have the game to obviously be there."

Roddick's opponent in the semifinals tomorrow will be No. 4 Nikolay Davydenko, who beat unseeded Janko Tipsarevic 6-2, 6-1.

To beat Federer for the first time since 2003, Roddick had to be at his best, especially down the stretch. He fell behind love-30 serving at 3-3 in the final set, then won the next 11 points, five of them thanks to unforced errors by Federer.

Twice during the stretch Federer shanked shots.

"I figure I was due," Roddick said. "He hadn't missed a ball in a crucial moment for about six years against me. I figured the law of statistics had to come my way eventually."

Federer lost serve at love to fall behind 5-3, losing the only break point he faced in the match. Roddick closed out the victory with a 140-mph service winner, and he served well throughout, hitting 18 aces — including two in the tiebreaker.

That helped him end a streak of 10 consecutive sets lost against Federer.

"I always said it's tough to play against Andy — you know, that serve," Federer said. "He's always going to have a chance. That's why I'm quite amazed about my record against him."

Roddick improved to 1-0 against Federer since his recent engagement to swimsuit model Brooklyn Decker, who watched from the stands.

"Being happy and content off the court is only going to help in my mind," Roddick said. Seeded sixth, he's two rounds from his second Key Biscayne title.

Williams toiled for nearly three hours to reach match point against Kuznetsova, then hit two more shots and raised a fist, tired but triumphant. On a muggy, 85-degree afternoon, Williams' newfound fitness may have made the difference.

She arrived for this year's tournament a slimmer Williams, joking after one win that this is the best she has looked since 1982. She was then an infant.

Her recent rigorous training regimen paid off in a semifinal that became a test of stamina.

"I don't feel tired or sore or anything," Williams said 90 minutes after the match. "I feel like I'm really fit. I've been working really hard."

In the final set, Williams broke for a 5-3 lead when she skipped a shot off the net cord, leaving Kuznetsova with an awkward backhand she dumped into the net.

Williams slammed her 12th ace to reach match point, and Kuznetsova then sailed a lob long.

"Serena was a bit fresher," Kuznetsova said. "In the end she was moving well and serving incredibly hard."

Williams' home-court advantage may have been a factor, too. She grew up in nearby Palm Beach Gardens.

"Serena, this is your house," one fan yelled.

Her 46-5 record at Key Biscayne suggests he's right.

"I love playing here," Williams said. "All my friends come out. I promised tickets for Saturday, so I was like, `I can't lose. I've got to stay in there a little bit longer.' It was mainly my motivation."

Jankovic barely survived her opening match. In the third set against Sofia Arvidsson, she overcame a 5-1 deficit, then saved five match points in a tiebreaker.

"It was a good thing for my confidence to get through the match in that way," Jankovic said. "Playing the way I'm playing now, it's really amazing, so I just hope to continue like this."

She has won every set in her past four matches to reach the final for the first time. Jankovic is 3-2 against Williams, including a victory in their most recent meeting at this year's Australian Open.

"I know how to play against her and know what to expect from her," Jankovic said. "But I also know that I really have to play good tennis in order to win, because she's a great champion and a tough opponent to beat."