* Sixth seed Del Potro upset
* Djokovic, Murray advance
* Hingis returns, Li into fourth round (Updates at end of day with Serena winning)
NEW YORK, Aug 30 (Reuters) - Lleyton Hewitt pulled off a stunning five-set win over Juan Martin del Potro at the U.S. Open after a classic late-night thriller between the two former champions on Friday.
Twelve years after he upset Pete Sampras to win the U.S. Open, the 32-year-old Hewitt showed he still has plenty of fight left in him as he floored Del Potro 6-4 5-7 3-6 7-6 (2) 6-1 in a four-hour slugfest at Arthur Ashe Stadium.
"I don't know how many years I have left in me so I was just hanging to get back on this court again," said Hewitt, who also won Wimbledon in 2002 but has not tasted grand slam success since and tumbled to 66th in the world rankings.
"This is why I still play, to have moments like this."
Del Potro, who beat Roger Federer in the 2009 U.S. Open final and was among the favourites to win this year, had his chances but ran out of gas, unable to keep up with one of his childhood heroes, eight years his senior.
"Tonight was a really tough battle for both but in the end he played better," the sixth seed said. "He's a great champion and a great fighter."
Del Potro became the biggest casualty of this year's championship, leaving the top half of the draw seemingly at the mercy of Novak Djokovic and Andy Murray, who both overcame wobbles to win on Friday.
Murray experienced a minor hiccup, dropping his first set of the tournament, but recovered quickly to beat Argentine baseliner Leonardo Mayer 7-5 6-1 3-6 6-1.
If there were any heart flutters amongst his army of supporters, who queued up outside Louis Armstrong Stadium just to catch a glimpse of the 26-year-old, they were short-lived as the defending champion rolled up his sleeves and got back to work.
"It was a very tough match," said Murray, whose next opponent is Germany's Florian Mayer. "Both of us did a lot of running."
Djokovic kept his perfect record intact after surviving an early test from Germany's Benjamin Becker on a baking hot day at Flushing Meadows, where gusting winds provided little relief from the heat and made it hard for the players to hit cleanly.
The world number one saved two set points in the opening set before rebounding to win 7-6 (2) 6-2 6-2 and advance to the next round against Portugal's Joao Sousa, a five-set winner over Jarkki Nieminen of Finland.
"It was a struggle," Djokovic said. "It was a lot of unforced errors, very windy conditions. You couldn't really read and kind of predict where the ball is going to go, so you have to be very alert."
The spectator lines were even longer at court 17 when Martina Hingis made her comeback to grand slam tennis 16 years after she won the U.S. Open and six years after she retired for a second time.
The former world number one was granted a wildcard entry to the doubles with Daniela Hantuchova but the pair fell at the first hurdle, losing 6-3 7-5 to defending champions Sara Errani and Roberta Vinci.
For Hingis it was a return to forget as she finished with back-to-back double faults to close out the match then lost a mixed doubles match, although the 32-year-old said she wanted to keep going.
"I think it's because I just played a lot and my body's not used to it," she said. "I feel like I'm playing well but my body is screaming, 'What are you doing to me?'"
Meanwhile, the current world number one, Serena Williams, steamrolled her way into the fourth round of the women's singles with a routine 6-3 6-1 win over Yaroslava Shvedova that didn't finish until after 1 a.m. on Saturday.
"I don't think I've played this late," said the 31-year-old.
She faces a tricky next assignment against Sloane Stephens, who beat her at the Australian Open in January. Stephens earned her place in the fourth round by beating Jamie Hampton 6-1 6-3 in a battle between two of America's brightest young prospects.
China's Li Na fired down 11 aces to defeat British teenager Laura Robson 6-2 7-5 to reach the fourth round while the ultra-consistent Agnieszka Radwanska wore down Russia's Anastasia Pavlyuchenkova 6-4 7-6 (1) to continue her impressive streak at this year's grand slams.
Her next match is against Russia's Ekaterina Makarova, who eliminated Wimbledon runner-up Sabine Lisicki of Germany 6-4 7-5. If she wins, the third-seeded Pole will become the only woman to reach the last eight at all four grand slams this year.
"Well, of course I will do everything to make the quarter-final this year," Radwanska said. (Editing by Nick Mulvenney)