KEY BISCAYNE, Fla. — Venus Williams sagged in the wooden changeover chair as if it were a cushy recliner, her head tilted back, her tank on empty.
She rose slowly to play the final two games and lost both, but grinned as she walked to the exit, perhaps because she was headed for a well-deserved rest.
Weary after winning three consecutive three-set matches, Williams was eliminated Wednesday by Agnieszka Radwanska in the quarterfinals of the Sony Ericsson Open, 6-4, 6-1.
"It was disappointing not to be able to feel my best today," Williams said. "I was able to keep it close in the first set and try different strategies, but it was definitely a mental battle, and today I didn't conquer the mental part of it."
Top-ranked Victoria Azarenka lost for the first time this year when she was beaten by Marion Bartoli, 6-3, 6-3. Azarenka had won 26 consecutive matches, a streak that dated to the beginning of 2012.
In the men's quarterfinals, No. 2-seeded Rafael Nadal continued his bid for his first Key Biscayne title by beating No. 6 Jo-Wilfried Tsonga 6-2, 5-7, 6-4. His opponent Friday will be 2009 champion Andy Murray, who overcame an upset stomach and early deficit to beat No. 9 Janko Tipsarevic 4-6, 6-3, 6-4.
Williams' serve lacked its usual velocity, and by the final game she wasn't even chasing shots in the corner. The tournament was her first since the U.S. Open last August, when she withdrew after being diagnosed with a fatigue-causing autoimmune disease.
"It has been a great tournament," she said. "Obviously I'm disappointed and would have liked to have gone further, but it's a great start."
Eager to keep points short in the 85-degree sunshine, Williams charged the net often but frequently had to lunge for shots. She committed 38 unforced errors to 10 for Radwanska, and won only five of 26 points on her second serve.
"It was pretty hot out there," Radwanska said. "Maybe that was why she didn't look that good."
Two Williamses were eliminated in 18 hours. Younger sister Serena lost Tuesday night to former No. 1 Caroline Wozniacki.
Current No. 1 Azarenka was off to the best start on the women's tour since Martina Hingis went 37-0 to begin 1997. An unreliable serve and atypical mistakes from the baseline doomed her against Bartoli.
"I'm a human, not a superwoman, and I wish I could be, but I'm not," Azarenka said with a smile. "I feel disappointed, sure. Who wouldn't be after the loss? But what I've done in the last couple of months, I have to be really proud of myself."
The steady Bartoli took charge of rallies by playing inside the baseline.
"Sometimes I took the ball extremely early," she said. "It was almost table tennis."
Seeded No. 7, Bartoli advanced to the Key Biscayne semifinals for the second time in three years. She'll play No. 5 Radwanska on Thursday night.
The other semifinal will be between Wozniacki and Maria Sharapova, a three-time Key Biscayne runner-up.
Venus Williams needed a wild card to enter the tournament because she's ranked 134th. The three-time Olympian's goal has been improve her ranking enough to qualify for the London Games, and she's projected to climb next week back into the top 90.
In the past, the cutoff for making the Olympics has been around 68th. Teams will be chosen based on rankings in early June.
Radwanska, ranked a career-best No. 4, advanced to the Key Biscayne semifinals for the first time. She's 0-4 this year against Azarenka and 24-0 against everyone else.
She beat Williams in 2006 but had since lost five consecutive meetings.
"Of course she had some time off last couple of months," Radwanska said. "But she's still a great player. I really had to play very well today to beat her."
Williams overcame a match point in the third round against Aleksandra Wozniak on Sunday night, and said she was up until 4 a.m. afterward. She recovered to beat No. 15-seeded Ana Ivanovic on Monday.
Williams said her ailment requires her to save all her energy for tennis, but she strayed from that approach with a day off Tuesday.
"I should have rested more probably," Williams said. "After a while you start to feel like maybe everything's behind you. I definitely learned maybe if you're doing something right, don't change it."
Her energy reserve appeared low from the start of the quarterfinal, and she lost the first seven points. Radwanska took advantage of Williams' lethargy by hitting several drop shots for winners and pouncing on weak second serves.
When Williams had a chance at an easy overhead, she walked up to the ball flat-footed. Facing break point early in the second set, she mustered only a 72-mph first serve, well below her norm of 110 or more.
Williams lost the final six points, dumped her last shot into the net and began looking ahead to the clay-court season. She plans to play next week at Charleston, S.C.
"I've definitely learned a lot about ways maybe I could prepare during the tournament," she said.