KEY BISCAYNE: Once roommates on grade-school tennis trips, Kim Clijsters and Justine Henin became Grand Slam champions, claimed the No. 1 ranking and played each other nearly two dozen times before calling it a career.

Now the friendly rivalry is out of retirement, and the two Belgians will meet once more Thursday night in the semifinals of the Sony Ericsson Open.

"It has always been special, and it will always be special," Henin said. "Because Kim and I grew up together; we arrived on the tour almost at the same time; we played good at the same time; we retired at the same time; and we came back at the same time."

Henin is only four tournaments into her return from a 20-month retirement, yet there's scant evidence of rust. She earned a berth in the semifinals by overtaking No. 2-seeded Caroline Wozniacki on Wednesday, 6-7 (5), 6-3, 6-4.

Clijsters retired, married and became a mother before returning to the tour last August. She advanced to the semifinals by beating No. 9 Samantha Stosur 6-3, 7-5.

"Fun to see the two comebacks doing well," Clijsters said.

Three-time champion Venus Williams will play No. 13-seeded Marion Bartoli in the other semifinal Thursday.

With top-ranked Roger Federer no longer looming in the men's draw, Andy Roddick will face Rafael Nadal in the semifinals Friday. Roddick advanced by beating No. 33-seeded Nicolas Almagro 6-3, 6-3, while Nadal eliminated No. 8 Jo-Wilfried Tsonga 6-3, 6-2.

Nadal, seeded fourth, saved all eight break points he faced. Roddick, seeded sixth, committed only 10 unforced errors in 110 points, and he has yet to drop a set in four rounds.

"When you play a lot of matches and kind of play a high level, it feels like everything kind of slows down a little bit," Roddick said. "Muscle memory takes over a little bit more. I think I'm at that stage right now. Unfortunately with tennis, you have to start over every day."

Federer lost to No. 16-seeded Tomas Berdych 6-4, 6-7 (3), 7-6 (6) in a fourth-round match that ended at 12:16 a.m. Wednesday. The upset could be a break for Roddick, who is 2-19 against Federer.

"Obviously you're surprised any time he loses," Roddick said. "But that's tennis. That's why you play."

Roddick trails Nadal 5-2, but has split their four matches on hard courts.

Henin leads her rivalry with Clijsters 12-11. They've met only once since 2006 — at Brisbane in January, which marked Henin's return to the tour. Clijsters saved two match points and won.

"That was one of the funnest, most exciting matches I think that I've played in my career," Clijsters said. "Hopefully we can both play the same kind of match level and intensity."

Henin was ranked No. 1 when she quit the tour in May 2008. She announced her comeback last September, soon after Clijsters won the U.S. Open, her third tournament following more than two years in retirement.

Henin received a wild card to enter Key Biscayne, and she has negotiated a tough path to the semifinals, beating four seeded players in a row. That makes her 16-3 since coming back, and she'll move back into the top 30 in next week's rankings.

"I enjoy my game much more than in the past," she said. "Every win has a very sweet taste at the moment."

The latest was sweet because Henin had to rally. She received treatment for soreness in her back midway through the second set but said the injury wasn't serious, and she summoned her best tennis down the stretch.

Henin lost only two of 22 service points in the final set, closing out the 2-hour, 45-minute match with a love game.

"I didn't play for two years, so it's good to be on the court a little bit longer," she said with a smile.

Clijsters took less than half as long to win her quarterfinal, and she has yet to lose a set in the tournament. She improved to 7-2 against top 10 players since her return.

"I'm going to try to hopefully raise my game a little bit more, because I know that I have to play some of my best tennis if I want to beat Justine," she said.

A two-time Grand Slam champion, Clijsters won Key Biscayne in 2005. Henin, the winner of seven major championships, seeks her first Key Biscayne title.