Key Biscayne, Fla. ? Tape took care of Venus Williams’ sprained right foot. There was no remedy for the shaky serve and late fade.

Williams was injured during the second set Wednesday, but sloppy shotmaking hurt her more, and she lost to Elena Dementieva 6-3, 5-7, 7-6 (3) in the quarterfinals of the Nasdaq-100 Open.

A three-time champion at Key Biscayne, Williams came up short in her bid to reach a semifinal for the first time since Wimbledon in July. Dementieva survived a match point and erratic play of her own to win the tense but unsightly 21/2-hour duel.

“It was a very difficult, long, but I think interesting match,” the No. 5-seeded Russian said.

The match had all the characteristics of a Venus loss: She double-faulted 11 times, committed 51 unforced errors and played hurt.

Williams took a timeout for treatment when she rolled her right foot in the final game of the second set. She actually played better when the match resumed, winning 14 of the next 17 points to take a 3-0 lead in the last set. But serving for the match at 5-3, Williams hit four double-faults, including three in a row and lose the game.

“It was sunny and the wind kept blowing, and my toss got crazy,” she said. “That was not good.”

There were other missed chances by Williams. She held a match point at 6-5 but sailed a forehand long, then committed five unforced errors in the tiebreaker.

Slowed by injuries since last summer, Williams has slipped to No. 17 in the rankings and has played only 10 matches in four events this year. Seeded second at Key Biscayne, she was unable to take advantage of a field missing the world’s five highest-ranked players.

Dementieva advanced to the semifinals, where she’ll play fellow Russian Nadia Petrova. Top-seeded Serena Williams will play No. 25 Eleni Daniilidou in the other semifinal.

In men’s play, No. 3-seeded Guillermo Coria became the first semifinalist by beating unseeded Nicolas Kiefer 6-3, 6-3. Coria will play Friday against No. 21 Fernando Gonzalez or unseeded Andrei Pavel.