"I just couldn't find my rhythm on the serve today, which was surprising," Federer said. "It's a bit frustrating, but Kei did well to stay with me. He was more consistent in the second and third, and in the end it's to his credit."
Li Na became the first Chinese woman to reach the Key Biscayne semifinals when she beat Caroline Wozniacki 7-5, 7-5. Li's opponent Thursday night will be Dominika Cibulkova, who 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.
Six-time champion Serena Williams will play five-time runner-up Maria Sharapova in the other women's semifinal. Williams has beaten Sharapova 14 consecutive times.
Murray won the Key Biscayne title in 2009 and 2013 but wasn't at his best against Djokovic. The Scotsman 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 call that made one point stand out from the other 125. Djokovic had won praise for his sportsmanship when he conceded a point following an erroneous call in his match Tuesday, but against good friend Murray, 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."
And Djokovic said the call was the umpire's to make.
"It's not my fault," he said. "I mean, I was never lying on the court. I always try to be fair to whoever I play against."