Updated

By Mark Lamport-Stokes

INDIAN WELLS, California (Reuters) - Britain's competition-rusty Andy Murray tumbled out of the Indian Wells ATP tournament after being upset 7-6 6-3 by American qualifier Donald Young in the second round on Saturday.

World number five Murray, runner-up here in 2009, lost the opening set tiebreaker 7-4 and was then broken twice by his 143rd-ranked opponent in the second.

"It's by far the biggest win of my career," a beaming Young said courtside after being roared on by a partisan crowd on the second stadium court.

It was the left-hander's first victory over a top-10 player in four attempts and earned him a place in the third round of the elite Masters Series event against Spaniard Tommy Robredo, a 6-3 3-6 7-5 winner against Germany's Mischa Zverev.

"Most of it was not great today," a subdued Murray told reporters after an erratic display in late afternoon sunshine at the Indian Wells Tennis Garden. "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."

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Murray had previously competed in only two tournaments this year, losing to Serb Novak Djokovic in the Australian Open final in January before being beaten by Cypriot Marcos Baghdatis 6-4 6-1 in his first match in Rotterdam last month.

The Scot was then forced to pull out of the Dubai Open which followed, citing a wrist injury, but he has since fully recovered and has just completed 10 days of training in Miami.

"I started the match well and then, when it got close to the end of the first set, I didn't do anything particularly well," said the 23-year-old Murray after serving up eight aces and four double faults.

"The crowd were for him. He started playing better. I didn't find my way back into the match."

Asked to assess the long-term potential of the 21-year-old Young, Murray replied: "I think he's good, but I don't know exactly how good.

"The toughest thing in the men's game for guys to break through has been the consistency. That's what you need to do if you want to try to get close to the top of the game. That's what Donald has to try and do."

(Editing by Frank Pingue)