KEY BISCAYNE, Fla. -- Martina Hingis, her legs cramping, needed help from a trainer Tuesday to survive an upset bid by Serena Williams at the Lipton Championships.

In

Martina Hingis returns a second-set serve from unseeded Serena Williams. (Reuters)

the unusual position of facing a player younger than she is, Hingis overcame a 5-3 deficit in the third set and two match points to edge Williams 6-3, 1-6, 7-6 (7-4) in the quarterfinals.

After holding serve in a long, harrowing game to trail 5-4 in the final set, Hingis called for a trainer and received three minutes of treatment.

"Today there was a lot of running," the top-ranked Hingis said. "I needed the trainer. I had a good reason. I probably should have done it before."

IF THERE WAS gamesmanship involved, the strategy worked: Hingis won the next two games for a 6-5 lead.

Williams regrouped and held serve to force the tiebreaker, but she lost the last three points, including the final two on unforced errors -- a forehand long, and an overhead pulled wide on match point.

"I think I tried too hard on a couple of shots," Williams said.

But the 16-year-old showed she's ready to join her 17-year-old sister, Venus, and Anna Kournikova, 16, as challengers to the reign of the 17-year-old Swiss star.

Kournikova beat Conchita Martinez in a fourth-round match 6-3, 6-0.

Hingis, the defending Lipton champion, advanced to the semifinals against the winner of Tuesday night's match between Venus Williams and Silvia Farina.

In men's fourth-round play, second-ranked Petr Korda squandered his chance to overtake No. 1 Pete Sampras in the rankings next week. Korda lost to Tim Henman 6-4, 6-4. Sampras was eliminated Monday by Wayne Ferreira.

FIFTH-RANKED Greg Rusedski lost to Thomas Enqvist 6-2, 6-2. Jeff Tarango rallied to upset sixth-ranked Yevgeny Kafelnikov 3-6, 6-3, 6-2.

Venus Williams watched from the players' section as her sister recovered from a slow start while Hingis became erratic and frustrated. Hingis lost her serve to fall behind 2-1 in the third set, then flung her racket, and it skidded 40 feet into the base of the umpire's chair as the crowd of 10,000 jeered.

Hingis rediscovered her groove from the baseline in the third set. Williams did most of the running and chased down a lot of shots, winning one 36-stroke rally that left Hingis panting and leaning on her racket.

But when the shots mattered most, Hingis was more consistent.

"It was just a great comeback," she said.