With the speed of a sports car, Ivo Karlovic's serves spun into Andy Roddick's body or skidded beyond his reach. One was smacked so hard, it bounced into the top tier of the grandstands. Another zoomed toward Roddick's head with such velocity that he ducked, rather than swing at it, as if a shotgun had been fired in his direction.

Roddick was so confounded by the spins, angles and pace of Karlovic's serves during yesterday's quarterfinal that at one point he turned to the grandstands, where fellow touring pro Nicolas Massu was sitting, and asked, "Are you getting any better read on this than I am?"

After nearly two hours of flailing away under the withering afternoon sun at Washington's William H.G. FitzGerald Tennis Center, Roddick survived Karlovic's barrage of 29 aces to pull out a 6-7 (9-7), 7-5, 6-4 victory and advance to today's semifinals of the Legg Mason Tennis Classic.

The top-seeded Roddick will face No. 13 seed Paradorn Srichaphan, who booked his semifinal spot with a 6-4, 7-5 victory over Luis Horna of Peru earlier in the day.

Today's other semifinal will pit James Blake, the tournament's 2002 champion, against 10th seed Tomas Berdych of the Czech Republic.

Blake breezed past Frenchman Arnaud Clement, 6-3, 6-2, in 1 hour 11 minutes, flashing the improved serve (eight aces) and more potent backhand that he developed during his recovery from illness and injury last season. The victory sends Blake into his first semifinal in two years and delighted Washington tennis fans, who packed the stands for his match.

Meantime, on the grandstand court, Berdych had a tougher time with American wild card Bobby Reynolds, who forced two tiebreakers before falling, 7-6 (7-5), 7-6 (11-9). The match ended on a double fault by Reynolds, and Berdych, in a gesture of sportsmanship, immediately turned to the stands and put his finger to his lips, requesting fans not cheer his opponent's costly error.

With the tournament's second through ninth seeds out of the event, Roddick is a heavy favorite to win his second Legg Mason championship and fourth title of the year. But for stretches, the towering Karlovic looked as if he would deprive him of the chance.

Said Roddick, who holds the record for the sport's fastest serve and had 19 aces in the match: "I don't think you'll hear me say this often, but I don't think my serve compares to his. You can get a racket on mine."

At 6 feet 10, Karlovic has the leverage to crank a monster first serve. His legs are so long he can follow his serve to the net in one or two strides. And once he's at net (picture kneecaps nearly level with the net cord), his reach is so huge that it's tough to rip passing shots by him. So on those fortunate occasions when Roddick got Karlovic's serve in play, the task of winning the point remained monumental.

"When he gets up on top of the net, you feel like you're kind of hitting into a wall," Roddick said. "There's not a whole lot of area to play into."

What ultimately bailed out Roddick, in a match that was decided by a handful of points, was his edge in fitness, agility and quickness. Karlovic may have the better serve (though the Croatian chuckled when informed Roddick had said so), but Roddick is the better returner, with more flexible knees and snappy reflexes. While Karlovic never managed a single break point on Roddick's serve, Roddick broke Karlovic twice -- once in the second set and again in the third. And that, combined with Karlovic's obvious fatigue in the third set, was the difference.

"It was hot, and in the third set, I had some problems in my legs," said Karlovic, who played both singles and doubles Thursday. "In the end, if you lose, it's not good. But at least it was more difficult for him than last time."

Roddick won their only previous meeting in straight sets, earlier this summer on grass in the Queen's tournament tuneup before Wimbledon. Then, he never managed to break Karlovic's serve but needed two tiebreakers to settle the contest.

And that's how their first set was decided yesterday. Roddick could hardly have served better, surrendering just three points on his six service games. The score knotted at 6, they slugged it out in a tiebreaker, swapping mini-breaks and aces until Roddick coughed up his first double fault of the match to lose it, 9-7.

In the second set Roddick tried standing farther back to receive Karlovic's serve, hoping that the extra split-second would help him gauge it. But the Croatian placed it so shrewdly that Roddick could have backed up to Silver Spring and still not had time to position his feet or racket face.

Finally, with Karlovic serving at 5-5, Roddick broke serve -- 1 hour 20 minutes into the match. He did it by blasting low groundstrokes as Karlovic charged forward, which forced the Croatian into awkward half-volleys that he drove into the net to lose three points.

The service-break was like a transfusion for Roddick, who served out the set to level the match. He clenched his fist, skipped to his seat at the changeover, whipped on a fresh shirt and then bounced back up, ready to start the third set before time was called.

Karlovic showed no emotion, but his legs betrayed his fatigue as the third set wore on.

James Blake, the 2002 champion, cruises past Arnaud Clement of France. He plays Tomas Berdych next.Andy Roddick shows his confidence after finally breaking the huge serve of Croatia's Ivo Karlovic.