KEY BISCAYNE, Fla. — Maybe it was the unpredictable wind or the unreliable forehand or the months of unsatisfactory results. Perhaps it was the sweltering mid-afternoon sun.

Whatever the reason, Roger Federer reached his boiling point Friday. He raised his racket over his head and slammed it to the concrete, a shocking outburst from the five-time winner of the ATP Tour's sportsmanship award.

The tirade came during a 3-6, 6-2, 6-3 loss to Novak Djokovic in the semifinals at the Sony Ericsson Open — the latest evidence Federer's in meltdown mode.

"It has been a tough last year or so," a red-eyed Federer said shortly after the match.

He fell behind 2-love in the third set, and when he netted an easy shot to start the next game, he mangled his racket with his most forceful forehand of the day.

"I was just frustrated," Federer said. "Didn't feel great. It's just a natural thing I did."

Pitching a fit failed to help. Federer lost the next two games as the match slipped away despite support from a sympathetic crowd.

The beneficiary was the No. 3-seeded Djokovic, winner of the 2007 title. He'll play in the final Sunday against the winner of the semifinal Friday night between No. 4-seeded Andy Murray and No. 6 Juan Martin del Potro, who upset top-ranked Rafael Nadal in the quarterfinals.

Top-ranked Serena Williams will bid for a record sixth Key Biscayne title Saturday against No. 11 Victoria Azarenka.

Federer was betrayed by his forehand, long touted as the game's best. It was the biggest culprit during the match's pivotal stretch bridging the second and third sets, when he lost 24 of 28 points and seven consecutive games.

Djokovic said his tactics contributed to Federer's frustration with his forehand.

"I tried to change pace and play fast, and then play a bit slower with more spin," Djokovic said. "He usually expects a ball on the backhand side, because that's where the players try to play him. But I'm not giving him any consistency on one side so he can really be comfortable."

Federer shanked at least half a dozen forehands, sending one 40 feet straight up. He blamed the breeze, a staple at Key Biscayne.

"I definitely struggled with my timing," he said. "You kind of try hard, and then it's just not working. Today it was different just because there's so much wind. Once you start feeling bad, it's kind of tough to regroup."

Federer had a reputation for tirades in juniors, and he broke a racket in anger in the 2005 Key Biscayne final against Nadal. But that's the last such tantrum he could remember, and he has long been considered the tour's model citizen.

So it was surprising when he kicked a ball in frustration after an errant forehand. Two games later Federer took out his frustration on his racket, which snapped at his feet.

The startled crowd jeered at first, then reconsidered its reaction as he trudged to his chair and unwrapped a new racket. When he walked back to the baseline, fans roared.

Djokovic had his back to the outburst.

"Look, it's obvious frustration," Djokovic said. "I just tried to keep my focus. I was on a roll in this period of the match, so I just tried to continue on playing patiently and win the match."

Even with fans firmly in Federer's corner down the stretch, his forehand continued to misfire, including twice in the final game before he sailed a backhand long on match point.

Federer has endured wrenching defeats in recent Grand Slams, including a loss in February at the Australian Open that left him sobbing. But he has also struggled in Masters events, the ATP Tour's most prestigious tournaments aside from the majors.

With the Key Biscayne defeat, he came up short of a title at his 13th Masters tournament in a row since August 2007. The drought is one reason his 4›-year reign atop the rankings ended last summer, when he was overtaken by Nadal.

Federer denied he's feeling a lot of pressure to win.

"I haven't been winning 20 tournaments in a row, so nobody expects me to win really," he said.

Federer now heads to Europe to play on clay, his least-favorite surface. Or maybe it's now hard court, where he failed to win a title during the January-to-March season.

"My game never really clicked," Federer said. "Thank God the hard-court season is over."