Andy Murray serves against Novak Djokovic on day ten of the Sony Open at Crandon Tennis Center, Miami, March 26, 2014. [Photo/Agencies]

KEY BISCAYNE, Florida - Novak Djokovic benefited from an erroneous call and claimed he didn't realize he had broken the rules. When Andy Murray walked to the net and challenged him about it, Djokovic responded with a shrug and a sheepish smile.

Murray lost the dispute, and the match. The pivotal ruling against the defending champion helped Djokovic win 7-5, 6-3 in the quarterfinals of the Sony Open.

Roger Federer was up a service break twice in the second set but lost to No 20-seeded Kei Nishikori 3-6, 7-5, 6-4. Nishikori will play Djokovic on Friday.

The point that aggravated Murray occurred at the start of the 12th game, when he was serving at 5-6. Djokovic charged forward to volley a short ball and hit it for a winner.

Murray argued - and TV replays confirmed - he should have been awarded the point because Djokovic's racket was on the far side of the net when he hit the ball. Chair umpire Damian Steiner declined to change his call, and such rulings can't be appealed for video review.

Murray briefly discussed the matter with Djokovic.

"I went and asked Novak, and he told me he was over the net," Murray said.

"Look, it might be my mistake," Djokovic said. "I think I crossed the net with the racket. I didn't touch the net. Maybe the rule is that you are not allowed to pass on his side with the racket. I'm not sure. You tell me."

That is indeed the rule. Djokovic said that if he had understood it correctly, he would have conceded the point.

A flustered Murray committed unforced backhand errors on the next three points to lose the game and the set.

"Obviously, that distracted him mentally, and after that he gave the set away," Djokovic said.

Murray took a lead in the second set but lost the final four games and the last 12 points. Afterward, he downplayed the bad call.

"I'm not angry," he said. "It maybe had a slight bearing on that game, but I was still up a break in the second set."

Nishikori will be playing in only his second ATP Masters 1000 semifinal, and his first in 2 years.

"I thought I really played well, especially in the third set," Nishikori said. "I was hitting balls deep and striking well. Everything was going well."

Federer was the runner-up to Djokovic at Indian Wells two weeks ago, and said he's pleased with his start to the year despite the latest loss.

"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."

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."

No 7-seeded Tomas Berdych advanced to Friday's semifinals, beating Alexandr Dolgopolov of Ukraine 6-4, 7-6 (3). Berdych was the Key Biscayne runner-up in 2010.