Mardy Fish punishes out-of-sorts Andy Murray in Florida
Murray has struggled for form since reaching the Australian Open final in January
Defending champion Andy Murray was dumped out of the Sony Ericsson Open in the second round with an error-strewn 6-4 6-4 defeat by Mardy Fish in Miami.
Murray, who had a first-round bye as the third seed, followed last week's Indian Wells quarter-final loss to Robin Soderling with a poor display.
And the defeat by Fish, ranked 101 in the world, means Murray will lose his number three ranking to Rafael Nadal.
"No excuses," he told BBC Sport. "It's down to me and getting my head right."
Murray, who defeated Novak Djokovic in last year's final, had been hoping to defend what remains his biggest career title to date in Florida.
Murray downcast after Florida exit
But since reaching his second Grand Slam final at the Australian Open in January, Murray has lost three of his seven matches.
He dropped the first set on Saturday despite leading 3-1 and 40-0, with American Fish capitalising on the Briton's inconsistent serve and increasingly wayward forehand.
Despite working on his game with coaches Miles Maclagan and Alex Corretja following his Indian Wells exit, Murray's display continued to be littered with unforced errors.
A series of screams at key points emphasised the world number three's frustrations as Fish clinched the first set after 46 minutes and took an early advantage in the second when he broke for a 2-0 lead.
And Fish sealed the match when he volleyed a Murray return into the open court, much to the delight of the home crowd.
"The last three tournaments have not been good," admitted 22-year-old Murray.
"I haven't been that tough on the court, which last year and at the beginning of the year in Australia was maybe my biggest asset. Since then it hasn't been good and it's something I'm going to need to improve."
He added: "Practise has been good, training has been going well, so no excuses.
"I obviously need to keep working hard and practising well but it's down to me getting right mentally.
"Obviously when you're focusing on negative things on the court you're going to play badly, and that's what I've done the last few weeks. I need to get way better."
A jubilant Fish, 28, enjoyed one of the best victories of his career, saying: "He's a Grand Slam finalist this year already, his results speak for themselves. It's a great win.
"He certainly didn't play the way he did in Australia - but I don't care."
Top seed Roger Federer won his opening match 6-3 6-3 against lucky loser Nicolas Lapentti, but the Swiss admitted that early defeats for Murray and Djokovic had concerned him.
"It worries me as well, being the top seed, seeing high seeds fall out of the tournament," he said. "I haven't even started playing yet, and there's two big names out of the tournament already."
Seventh seed Marin Cilic cruised into the third round with victory over Frenchman Stephane Robert 6-3 6-1, while ninth seed Fernando Gonzalez beat Lu Yen-Hsun of Taiwan 6-4 6-4.
Fifth seed Soderling dispatched Australia's Peter Luczak 7-6 (7-5) 6-0 and 10th seed Fernando Verdasco beat Israel's Dudi Sela 6-1 6-2.
But 31st seed Janko Tipsarevic crashed out following a 6-4 6-0 defeat by Philipp Petzschner.
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