Dominika Cibulkova erased three match points in the second set -- one when a call was overturned via replay -- and beat Agnieszka Radwanska 3-6, 7-6 (5), 6-3. In Thursday's semifinals, the No. 10-seeded Cibulkova will play the winner of the match Wednesday night between No. 2 Li Na and No. 11 Caroline Wozniacki.
Murray won the Key Biscayne title in 2009 and 2013 but wasn't at his best against Djokovic. The Scot committed five double-faults and 32 unforced errors, including a flurry down the stretch.
He took a 3-2 lead in the second set with his only service break, then double-faulted twice to give it right back.
But it all might have been different if not for the much-discussed call late in the first set. After Djokovic won the disputed point, an irritated Murray walked to the net to discuss the matter with his opponent, who smiled and shrugged.
Djokovic had won praise for his sportsmanship when he conceded a point following an erroneous call in his match Tuesday, but this time he left the verdict to the umpire.
A sideline reporter told Murray that TV replays showed he was correct about the call, and during the ensuing changeover he questioned Steiner.
"His racket was over the net," Murray said. "It's quite clear. You can see it on the replay."
"I have to make a decision at the moment," Steiner replied.
Any hard feelings on Murray's part didn't carry over after the match, when he shook hands with Djokovic and Steiner.
"It's a hard one for the umpire to call," Murray said. "Just frustrating."
Cibulkova won despite losing her serve 10 times, double-faulting eight times and committing 51 unforced errors. There were 11 breaks in the first 12 games of the match.
Cibulkova collapsed to her back in jubilation when Radwanska's final shot sailed out. The runner-up at the Australian Open in January, Cibulkova advanced to the Key Biscayne semifinals for the first time.
Former No. 1 Martina Hingis teamed with Sabine Lisicki and overcame three match points against Anabel Medina Garrigues and Yaroslava Shvedova to win 3-6, 7-6 (7), 10-7 and reach the semifinals in women's doubles.