* Del Potro upsets erratic Murray in three sets
* Will next face top seed Djokovic
* Djokovic seals victory in under an hour (Adds Murray quotes, detail)
INDIAN WELLS, California, March 15 (Reuters) - Erratic serving cost Andy Murray dearly as the U.S. Open champion was knocked out of the BNP Paribas Open quarter-finals 6-7 6-3 6-1 by Argentina's Juan Martin Del Potro on Friday.
Though the British world number three edged a close first set 7-5 on the tiebreak, he lost serve once in the second and three times in the third to make a premature exit from the ATP Masters 1000 event at the Indian Wells Tennis Garden.
Seventh-seeded Del Potro will next face world number one Novak Djokovic, who was barely tested as he demolished Jo-Wilfried Tsonga 6-3 6-1 earlier on Friday to extend his unbeaten run to 22 matches.
The first two quarter-finals were completed on Thursday, Rafa Nadal continuing his impressive comeback by thumping long-time rival Roger Federer 6-4 6-2 and Czech Tomas Berdych sweeping past big-serving South African Kevin Anderson 6-4 6-4.
"It was a tough match, I didn't serve particularly well," Murray told reporters after serving eight aces and eight double faults during a match lasting just over two-and-a-half hours.
"There were a lot of long rallies. Very warm conditions the first set or two, and sometimes on the serve if your legs are just a little bit tired you can miss serves.
"Timing might go a little bit off and you're not quite getting up to them. That's maybe what happened today."
Asked if he had played the game he wanted to play against the 6ft-6in (1.98 metre) Del Potro, Murray replied: "I could've served better and returned better, two pretty important parts of the game."
Del Potro, the 2009 U.S. Open champion, was delighted to beat Murray for the first time in five meetings on hard courts.
"I was close to beating him in 2009 in Montreal finals, but I did today," smiled the 24-year-old from Tandil after improving his record this year to 15-3. "I played my best match of the tournament.
"I was positive all the time, even when I lost the first set, a tough first set. In the end, I play my game. I be aggressive all the time... trying to make winners when I had the chance with my forehands."
On a sweltering afternoon at Indian Wells, Murray saved two break points in the first game of the match but there were no further opportunities for either player as a closely contested opening set went into a tiebreak.
A big forehand winner by the Scotsman earned him a 5-2 lead but he double-faulted and then netted a forehand, screaming to himself in disgust, as he let Del Potro pull back to 5-4.
The Argentine then hit a backhand long for Murray to earn his first set point at 6-4 and, though he squandered that with a wayward backhand, he clinched the set at the next opportunity on a backhand error by his opponent.
However, the Scot failed to take advantage and was broken to love in the second game of the second set after double-faulting. Del Potro served out to level the match, clinching the set with a leaping overhead smash.
Murray was again broken in the third game of the final set, after netting a forehand, and also in the fifth, after hitting a backhand long, to trail 1-4.
Unable to break the towering Argentine's serve, Murray's challenge faded quickly and the match ended when the Scotsman double-faulted for an eighth time.
Top seed Djokovic delivered a ruthless display against the eighth-seeded Tsonga in the first match of the day, breaking his error-prone opponent twice in each set and not dropping a single point on his own serve in the second set.
"I definitely feel very happy about this performance today," twice Indian Wells champion Djokovic said after improving his record for the season to 17-0 in a match lasting just 54 minutes.
"I thought Jo didn't play his best. He made a lot of unforced errors and his serve wasn't going well and that made my life a lot easier on the court.
"I really didn't care about how my opponent felt. I just tried to focus on the job I need to do, and my performance was really good."
The Serbian world number one will be looking to extend his stellar form, having not been beaten since last October when he was defeated by big-serving American Sam Querrey in the second round of the ATP Masters 1000 event in Paris. (Editing by John O'Brien)