Victoria Azarenka's 26-match unbeaten streak across the first three months of the year was ended on Wednesday by Marion Bartoli, who beat the world No. 1 in straight sets in the quarterfinals of the Sony Ericsson Open.
Azarenka's streak, which included her Australian Open title, was the best on the women's tour since Martina Hingis went 37-0 to begin 1997.
Bartoli's semifinal opponent on Thursday will be Agnieszka Radwanska, who beat a tired Venus Williams 6-4, 6-1.
In the men's quarterfinals, 2009 champion Andy Murray overcame an upset stomach and early deficit to beat No. 9-seeded Janko Tipsarevic 4-6, 6-3, 6-4.
Azarenka'd 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.
The other semifinal will be between Caroline Wozniacki and Maria Sharapova, a three-time Key Biscayne runner-up.
Williams was weary after winning three consecutive three-set matches in her comeback tournament. It was her first tournament since the U.S. Open last August, where she withdrew after being diagnosed with a fatigue-causing autoimmune disease.
"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."
"It has been a great tournament," she said. "I'm disappointed and would have liked to have gone further, but it's a great start."
Eager to keep points short in the heat, 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.
Radwanska, ranked a career-best No. 4, is 0-4 this year against Azarenka and 24-0 against everyone else.
Murray lost six straight games after leading the first set 4-2, but fought back to clinch a semifinal berth against the winner of Wednesday's late match between Rafael Nadal and Jo-Wilfried Tsonga.
"Today's match was really important for me to get through after how it was going, (fighting) back after being a break down a few times in the second set," Murray said.