KEY BISCAYNE, Fla. — The stadium erupted when Mardy Fish walloped the final shot of the match for a winner, and he let loose a jubilant roar of his own.

The American earned one of the biggest victories in his 10-year career yesterday at the Sony Ericsson Open, where he upset 2009 champion Andy Murray, 6-4, 6-4.

Fish repeatedly came through with big serves at key moments, while Murray went into a funk after he frittered away an early lead. The Scotsman was the Australian Open runner-up in January and was seeded No. 3 at Key Biscayne but failed to survive his opening match.

"He probably didn't play the way he did in Australia," Fish said. "But I don't care."

Murray confirmed he has been struggling the past two months and described the problem as mostly mental.

"It's purely down to me, what goes on inside my head," he said. "It doesn't matter how well you practice. You need to be tough in the matches."

Top-ranked Roger Federer held every service game and won his opening match by beating lucky loser Nicolas Lapentti, 6-3, 6-3. Federer, the Key Biscayne champion in 2005-06, has an inviting path to the final with Murray out. No. 2 Novak Djokovic was eliminated Friday.

Rafael Nadal will overtake Murray for the No. 3 spot in the next rankings, while Fish advances to the third round at Key Biscayne for the first time since 2003.

"I desperately wanted to play well here," said Fish, who is from Vero Beach.

While U.S. tennis fans could boast about the upset, their lone remaining hope on the women's side also advanced. Three-time champion Venus Williams reached the fourth round by beating Roberta Vinci 6-1, 6-4.

Williams is the only American who reached the final 32 in the women's draw.

Top-seeded Svetlana Kuznetsova beat No. 27 Agnes Szavay, 6-2, 6-3. But former No. 1 Ana Ivanovic's recent struggles continued with a loss to No. 6 Agnieszka Radwanska, 7-5, 7-5.