Andy Murray made an early exit at the BNP Paribas Open in Indian Wells as US qualifier Donald Young caused a major upset with a straight-sets victory.
The American, 21, won 7-6 (7-4) 6-3 in one hour and 34 minutes to leave Murray without a singles victory since the Australian Open in January.
And the defeat will raise fears of Murray suffering a similar slump in form to that of this time last year.
His next chance to get a win will come in Miami in a fortnight.
"Most of it was not great today," Murray said. "I didn't serve particularly well, I didn't move very well.
"Early on in tournaments when you're not hitting the ball well, running a lot of balls down and making your opponent play, you can sometimes get the job done.
"But I didn't move particularly well and he hit a lot of winners. He gained in confidence I guess from hitting more winners."
Murray's failure to overcome Young, the world number 143, came on the back of a first-round loss in Rotterdam last month and a heavy defeat by Novak Djokovic in January's Australian Open final.
Despite getting the first break of the match on Saturday, fifth seed Murray never looked comfortable and there were signs that he might be struggling with a knee problem.
It's by far the biggest win of my career
Whatever the reason, the Scot let Young back on level terms immediately at 3-3 when he framed a forehand high into the stands and he was unable to force another break point before the tie-break.
It was the more aggressive Young who dominated that exchange, moving 4-2 clear and wrapping up the set when Murray blazed a forehand wide.
Young, a former junior world number one, was gifted three break points at the start of the second set and fired a backhand winner to grab what could have been the decisive advantage, but nerves took hold and Murray was level within moments.
It did not signal a change in fortunes, however, with Young continuing to do all the pressing and Murray buckling under the pressure once again at 2-2, finding the net with a backhand on break point.
Young was not going to let his man off the hook a second time and he confidently served out the match, screaming in delight in his moment of victory.
"It's by far the biggest win of my career," said Young, who will play 25th seed Tommy Robredo in round three.
"I've had chances before but this is the first I've come through and didn't let my nerves get the better of me."
Murray must now concentrate on the doubles in which he is partnering his brother Jamie.
In other men's singles matches, Juan Martin Del Potro beat 14th seed and defending champion Ivan Ljubicic 5-7 6-4 6-2, while Ivo Karlovic upset sixth seed David Ferrer 7-6 (7-3) 6-3.
World number one Rafael Nadal later beat South African qualifier Rik de Voest 6-0 6-2.
The Spaniard now has two career wins over de Voest - their only previous meeting coming in 2003 at a Challenger event in France when Nadal was just 16.
"I think there have been a lot of things change, no?," Nadal said. "I am better player than in that moment. The worst thing is I am eight years older."
Nadal, who had a bye in the first round, will play either Ryan Sweeting or Juan Monaco in the next round.
Fourth seed Robin Soderling also progressed with a 6-3 7-6 (7-4) win over German Michael Berrer.
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