MELBOURNE, Australia — U.S. Open champion Juan Martin del Potro’s bid for back-to-back Grand Slam titles ended in a fourth-round, five-set loss to No. 14 Marin Cilic on Sunday.

The 21-year-old Argentine had an upset win over top-ranked Roger Federer in the U.S. Open final last September, ending the Swiss star’s streak of five straight titles in New York.
He was the first man since Novak Djokovic won the 2008 Australian Open to interrupt the Federer-Rafael Nadal domination of men’s Grand Slam titles, and started the season’s first major with the No. 4 seeding and among the favorites.
Nadal and Andy Murray held off two of the tallest men and biggest servers in tennis earlier on Rod Laver Arena to set up a quarterfinal meeting.
Nadal, who beat Federer in the final to win the 2009 Australian title, had a 6-4, 4-6, 6-4, 6-4 win over 6-foot-10 Ivo Karlovic of Croatia, the tallest man on the tour.
No. 5 Murray overcame 6-foot-9 American John Isner 7-6 (4), 6-3, 6-2 to reach the final eight in Australia for the first time. The 22-year-old Scot hasn’t dropped a set in four matches.
In an afternoon match at Hisense, Nadia Petrova advanced with another upset win, beating third-ranked Svetlana Kuznetsova 6-3, 3-6, 6-1 to reach the quarters.
Zheng Jie set another national mark by becoming the first Chinese player to reach the quarterfinals at Melbourne Park with a 7-6 (6), 6-4 victory over Alona Bondarenko.
Del Potro was pushed all the way in the 4-hour, 38-minute match against Cilic, who is the same age and had an identical 9-3 record at Melbourne Park. Cilic exacted a measure of revenge with the victory. He lost to del Potro in the fourth round here last year and in the quarterfinals to him at the U.S. Open.
Del Potro had been bothered by right wrist pain that forced him out of a warmup tournament at Kooyong and was undoubtedly weary from a five-set win over James Blake — the longest of the tournament in terms of total games (62) of games at 62.
He looked lethargic at times, and Cilic had enough opportunities — converting four of his 16 breakpoint chances — to advance to a quarterfinal against either former No. 1 Andy Roddick or 2007 Australian Open finalist Fernando Gonzalez of Chile.
Nadal was already looking ahead to a different style of match against Murray.
“Being in the quarterfinals is very good news for me ... I’m going to have a very difficult match against Andy,” Nadal said. “I want to play my game. ... If not maybe I will have a good flight home.”
Nadal only dropped serve once against Karlovic, to surrender the second set. In the third and fourth sets, he broke the Croat in the third games to get on top early.
He showed plenty of emotion after earning a breakpoint in the third set, screaming out “Vamos!” and spinning as he ripped his left elbow back and raised his left knee.
When he converted the break for a 2-1 lead, he made three-punch celebration.
“I just wait my chance. Be focused on my serve ... and wait the chance on the return,” Nadal said. “I was returning really well when I had the chance to touch the ball.”
Murray lost the 2008 U.S. Open final to Federer, his only trip to a Grand Slam championship match, and knows he’ll probably have to beat both Nadal and Federer to claim his first major here.
He rated Isner’s serve among the best in the game, so he was delighted to wear down the American after a crucial service break in the eighth game of the second set. He fended off breakpoint chances in the next game to serve out the set.
“It’s been good. I moved really well since the start of the tournament,” Murray said. “I didn’t drop a set yet ... not too much to complain about.”
The loss was Isner’s first of the year, which started with his first career ATP title at Auckland, New Zealand.
Petrova had two first-round exits in warmup events but has built momentum in Melbourne and was coming off a stunning third-round 6-0, 6-1 win over U.S. Open champion Kim Clijsters.
“Today was similar. I felt like my motivation, my concentration was on the same level,” said Petrova, who will meet the winner of the all-Belgian match later Sunday between former No. 1 Justine Henin and 2009 U.S. Open semifinalist Yanina Wickmayer.
The 27-year-old Russian doesn’t think she’s achieved her potential yet when it most counts. In her best runs, she lost French Open semifinals in 2003 and ’05.
“I know there’s still a lot to accomplish,” Petrova said. “There’s still a lot to let out. That’s what I’m really looking for.”
Zheng is still a win away from equaling her best performance at a Grand Slam.
The 26-year-old Zheng made a stunning charge to the Wimbledon semifinals in 2008, becoming only the second wild-card entry and first Chinese player to go that far at a major.
“I think Australia is lucky for me because I win the first Grand Slam doubles title in Australia,” said Zheng, who won the Australian Open and Wimbledon doubles titles in 2006 with Yan Zi. “So I’m so happy I’m in the quarterfinal of Australia also.”
She and No. 16 Li Na both reached the fourth round here, the first time two Chinese players have reached that stage at the same Grand Slam tournament.