Andy Murray surrendered a one-set lead to crash out of the Pacific Life Open 2-6 7-5 6-3 to Germany's Tommy Haas.
The British number one started strongly, breaking in the third and fifth games to take the opening set.
Murray's serve deserted him in the second and Haas stormed to a 4-1 lead, and although Murray recovered the break, the German broke again to level.
Murray then passed up five break points in the decider, and paid for it as Haas broke him to love and then served out.
The Scottish 20-year-old berated himself afterwards, saying: "If I had played a bit more aggressively in the third set - like I did in the first set - then I would have won the match.
"I got annoyed a couple of times but that was more because I was playing a bit defensive "
"I still think I was good enough to win the match but I didn't."
Murray had vowed to keep his temper under control against Haas having failed to stay calm in his third-round victory over Ivo Karlovic.
But it was another outburst in the final set against Haas that seemed to turn the tide for the German, who is making his comeback from shoulder surgery.
After missing a chance to break Haas at 4-3, Murray screamed out in frustration, but that only succeeded in firing up Haas who immediately broke the Briton to love.
The German set up two match points and Murray netted on the second of them to hand Haas victory.
606: DEBATE
Murray was at least happier with his on-court attitude than he had been after his win over Ivo Karlovic in the previous round.
"It was better than yesterday, that's for sure," he said.
"I got annoyed a couple of times but that was more because I was playing a bit defensive and wasn't quite finding a way to get on the offensive in the third set.
"But the mindset was way better today than it was yesterday. I was much calmer at the start of the match."
There was better news for the British number one's brother, Jamie Murray, who reached the semi-finals of the men's doubles with Belarusian playing partner Max Mirnyi.
The duo enjoyed a superb 5-7 6-4 (10-6) win over American top seeds Bob and Mike Bryan after a match tie-break.
The victory sets up a last-four meeting with Israeli duo Jonathan Erlich and Andy Ram.
~RS~q~RS~~RS~z~RS~36~RS~)