INDIAN WELLS, California (AP): Top-ranked Rafael Nadal struggled past big-serving Ivo Karlovic 5-7, 6-1, 7-6 (7) on Thursday to reach the BNP Paribas Open semifinals for the sixth consecutive year.

Nadal, who won on his third match point when Karlovic's forehand sailed long, set up a semifinal against Juan Martin del Potro on Saturday. The Argentine progressed when his opponent Tommy Robredo of Spain withdrew before the match due to an abductor muscle strain.

"The match was the first one that I felt really that even if I play well, I can lose," said Nadal, who will be aiming for his third title at Indian Wells. "That's always hard to go on court with this feeling."

Nadal led 5-2 in the tiebreaker before Karlovic tied it up with a backhand volley winner. The Croat came up with the same shot to spoil Nadal's first match point.

Nadal set up his second match point with a forehand that landed just inside the sideline, but his forehand error allowed Karlovic to tie it 7-all. Nadal's forehand winner in the corner gave him a third match point and he finally converted on Karlovic's error.

"You lose a set against Karlovic and you are under pressure the rest of the games," Nadal said. "In the third, I didn't have chances. His serve was unstoppable. In the tiebreak I was really nervous. I returned fantastic in the tiebreak, but I didn't put one first serve on the court."

Karlovic had hardly been playing like his No. 239 ranking. He beat three seeded players to reach the quarterfinals, and pushed Nadal before losing to him for the fourth time.

On the women's side, top-ranked Caroline Wozniacki reached the semifinals when Victoria Azarenka retired with a hip injury while trailing 3-0 in the first set.

Wozniacki next plays Maria Sharapova, who outlasted Peng Shuai 6-2, 5-7, 6-3 to reach her first semifinal of the year after missing the last five weeks with a viral illness.

Sharapova needed more than two hours to overcome Peng in a match that featured 13 service breaks. They traded breaks in the first four games of the third set. Serving at 3-2, Sharapova won the final four games of the match, clinching it with an inside-out forehand off Peng's second serve.

"I was playing really well for the first set and a half and then really let her back in the game," said Sharapova, who had 13 double faults. "She's a really aggressive player and puts a lot of pressure on from you the first point, and if you're not the one doing that it becomes dangerous. The most important thing is I got myself together and I focused and regrouped."

Wozniacki was up 2-0, 30-0 when Azarenka called for a trainer. The Belarussian lay on the court and had her upper left thigh massaged before she took a 10-minute medical timeout.

Azarenka returned with her left thigh wrapped. Wozniacki resumed serving and won the next two points to go up 3-0 before Azarenka signaled she was done.

They hugged at the net and Wozniacki patted Azarenka on the shoulder.

"Victoria is one of my best friends on the tour, and to see her in pain on the court and to see her get injured was not nice for sure," Wozniacki said.

During their match, a Japanese flag was tied to the chair umpire's tower and written on it in black marker were the words, 'Our thoughts are with you! Caroline and Victoria.' After Azarenka left, Wozniacki untied it and held it up in tribute to the victims of the recent earthquake and tsunami.

"It's just devastating to see what's going on in Japan," the Dane said. "We just wanted to show that we're thinking about them over there."

Wozniacki had asked her manager to locate a Japanese flag. He finally found one, two hours from Indian Wells.

"I said, 'You better take your car and drive over there and get that flag to me,"' she said, smiling. "Victoria helped me out, what to write. I wanted to do it so perfectly that my hand almost cramped when I was doing it."

By reaching the semifinals for her fourth straight tournament, Wozniacki guaranteed she will retain the No. 1 ranking through the upcoming two-week Sony Ericsson Open in Miami.

Robredo hurt himself in his straight-sets win over Sam Querrey, and an MRI on Wednesday night confirmed the injury, which will keep him out of the Miami tournament.

Del Potro has beaten three seeded players, including defending champion Ivan Ljubicic, to get this far. He's in the semis for his fourth straight tournament after injuries derailed him for much of last year.

INDIAN WELLS (California): Australian Open champion Novak Djokovic, Rafa Nadal and Roger Federer reached the quarter-finals at Indian Wells on Wednesday without dropping a set while eighth-seeded Andy Roddick bowed out.       

Djokovic extended his perfect record this season to 15-0 by pounding fellow Serb Viktor Troicki 6-0, 6-1 and Spanish world number one Nadal had to work a little harder before seeing off plucky Indian qualifier Somdev Devvarman 7-5, 6-4.       

Second-ranked Federer, seeking a record fourth title at Indian Wells, was tested to the full in a tight opening set by 18-year-old American Ryan Harrison before booking his place in the last eight 7-6, 6-3.       

"He was making it very hard for me," 16-time Grand Slam champion Federer said courtside after playing wildcard Harrison for the first time. "It was a very tricky match. I think he's going to have a great future."       

However, big-serving American Roddick was eliminated 6-3, 7-6 by Frenchman Richard Gasquet who clinched a gripping second set tiebreak 7-5 after his opponent had earlier been warned for racquet abuse.       

Gasquet will next face Djokovic, Nadal takes on Croat Ivo Karlovic, a 7-6, 6-2 winner against Spaniard Albert Montanes, and Federer will play fellow Swiss Stanislas Wawrinka, who upset seventh-seeded Czech Tomas Berdych 3-6, 6-4, 6-4.       

Although Djokovic needed five match points before sealing victory in 66 minutes, he outclassed his good friend and doubles partner with a superb display of sliced backhands, crunching top-spin forehands and delicate drop shots.       

With late afternoon shadows lengthening on the showpiece stadium court, the third seed ended the match with a rasping backhand winner down the line before saluting the crowd by raising both arms skywards.       

"I am playing probably the best tennis of my life these last three months," Djokovic, 23, said. "And it all started with the Davis Cup win (in December's final) where Viktor was a hero."       

Djokovic, who claimed his second Grand Slam crown at the Australian Open in January and his 20th ATP title at last month's Dubai championships, broke Troicki three times to sweep through the first set.       

He again broke his opponent in the fourth and sixth games of the second before the two Serbs treated the stadium crowd to the best rallies of the match in a protracted final game.       

Nadal, champion here in 2007 and 2009, broke Devvarman twice to win an erratic opening set and the second went with serve until the Spaniard again broke in the decisive 10th game.       

Devvarman endeared himself to the crowd watching the action on the showpiece stadium court with his gritty play and occasional winners but Nadal finally sealed victory with a sublime passing shot.        "For sure, he did well and had a very good tournament," the Spaniard said of the first Indian to reach the fourth round at Indian Wells.       

Gasquet, the 18th seed, raced into a 4-1 lead in his tiebreak with Roddick but the American clawed his way back to 5-5 before the Frenchman ended the match with a stunning backhand service return winner down the line.       

"It feels incredible for me to be in the quarter-finals here," Gasquet said after beating Roddick for just a second time in five meetings.        "I did well with my serve and my backhand, especially on match point. I had nothing to lose and it would have been difficult for me in a third set."       

Roddick, who was warned for racquet abuse by chair umpire Fergus Murphy when trailing 1-3 in the second set, applauded the Frenchman's performance.       

"He played really well," said the American, runner-up here last year. "He outplayed me from the first ball. He was obviously very confident."        Earlier, Spaniard Tommy Robredo and former US Open champion Juan Martin del Potro of Argentina booked their places in the last eight.       

Robredo, despite being hampered by a hamstring strain, crushed American Sam Querrey 6-1, 6-3 and Del Potro came from 1-6 down in the second set tiebreak to beat Philipp Kohlschreiber 7-6, 7-6.

In the women's quarter-finals, Yanina Wickmayer beat No. 10 seed Shahar Peer 6-3, 6-3 and No. 15 Marion Bartoli ended Ana Ivanovic's run with a 6-4, 7-6 (7-4) win. — Reuters