* Top seed Djokovic stunned by Del Potro
* Nadal reaches fourth final at Indian Wells
* Spaniard beats sixth seed in straight sets (Updates with Djokovic defeat, adds quotes, detail)
INDIAN WELLS, California, March 16 (Reuters) - While Rafa Nadal continued his impressive comeback to reach the BNP Paribas Open final, Novak Djokovic made a shock exit in Saturday's semi-finals as his winning streak of 22 matches came to an end.
The top-seeded Serb squandered a 3-0 lead in the final set before losing concentration and the match, 4-6 6-4 6-4 to Argentina's Juan Martin Del Potro, on a sweltering afternoon at the Indian Wells Tennis Garden.
Djokovic, a twice champion here, was broken twice in that third set as Del Potro booked his place in Sunday's final against world number five Nadal, who overcame Czech Tomas Berdych 6-4 7-5 in a semi-final of high quality.
"It happens," a gracious Djokovic told reporters after losing for the first time since his defeat last October to big-serving American Sam Querrey in the second round of the ATP Masters 1000 event in Paris.
"All the credit to my opponent today. He deserved to win because he was more composed in the important moments and he played the right shots.
"I didn't deserve to win today. You know, whenever I had chances, in the second or third sets, I throw them away with some unforced errors. My movement was poor, and I congratulate to my opponent."
Seventh seed Del Potro, who won his first and only grand slam title at the 2009 U.S. Open before being sidelined for eight months the following year after having wrist surgery, beat Djokovic for only the third time in 11 meetings.
"I was so close to lose because he got the chance to beat me but I got lucky in very important moments, very important points when I made fantastic winners on my forehands," said the 24-year-old from Tandil.
"It was my best match in this tournament, for sure," added Del Potro, who upset third-seeded Scotsman Andy Murray 6-7 6-3 6-1 in the quarter-finals.
The towering Argentine with the big serve and booming forehand lost a closely contested first set in just under an hour after being broken in the 10th game.
Though he lost serve twice more in the second set, he broke the Serb three times to level the match and then twice more in the third as he played steadier tennis from the baseline, his running forehand often making a telling difference.
Earlier, twice champion Nadal made light of his seven months out last year due to a knee injury as he outplayed the sixth-seeded Czech.
The Spanish left-hander, who returned to the ATP circuit only last month, broke his opponent once to take the opening set, then won a tense second set to improve his record this year to a career-best 16-1.
"For me this is totally unexpected to be in the final," smiled Nadal after winning the last four games to come from 3-5 down in that second set.
"But here we are today, and I'm very happy about all what happened the last month, especially the last three weeks.
"Not easy coming back after an important injury after seven months without playing a tournament and be back and winning against three top 10s like David Ferrer, like today against Berdych, the other day against Roger (Federer)."
Nadal swept aside Spanish world number four Ferrer 6-0 6-2 in the final of the Mexican Open two weeks ago and crushed long-time rival Federer 6-4 6-2 in the Indian Wells quarter-finals on Thursday.
"So coming back is certainly something amazing for me," the Spaniard said. "I feel very comfortable in this tournament. I feel very happy when I am here."
Twice Indian Wells champion Nadal, who has beaten Del Potro seven times in their 10 career meetings, including the last three matches, played down any suggestion he would be a firm favourite in Sunday's final.
"Del Potro is coming in fantastically well, winning in Rotterdam, playing a very good tournament in Dubai, and now being in final here," said Nadal.
"So it's going to be a very difficult final for me. But I don't have nothing to lose after seven months. I did much more than what I dreamed. Any result will be a fantastic week for me, a fantastic tournament." (Reporting by Mark Lamport-Stokes; Editing by John O'Brien)