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John Donegan, Associated Press
Rafael Nadal of Spain, left, argues with chair umpire Carlos Bernardes over a line call as he plays Tomas Berdych of the Czech Republic during their quarterfinal at the Australian Open tennis championship, in Melbourne, Australia, Tuesday, Jan. 24, 2012.

MELBOURNE, Australia — The challenge system is proving, well, challenging for the top men at the Australian Open.

No. 4-ranked Andy Murray asked for five line calls to be reviewed by video technology in his quarterfinal against Kei Nishikori on Wednesday and failed on every one. No. 2 Rafael Nadal and No. 6 Tomas Berdych challenged a combined 20 calls the previous evening and got eight right.

Nadal was infuriated at 5-5 in the first-set tiebreaker when he tried to challenge a call after he had returned a shot, only to be told by umpire Carlos Bernardes he was too late.

The Spaniard later explained that he was more upset that Bernardes hadn't overruled the call — television replays showed the ball was clearly long.

Nadal believes the challenge system, which allows players three incorrect challenges per set, has discouraged umpires from intervening on line calls.

"They are there to make something, not just to call 15-all or 15-30 all the match," he said. "That's the only thing I'm unhappy about."

He was a bit more pointed during the match when he said to Bernardes in Spanish: "You're not here as a spectator."

Nadal isn't doing too badly on his challenges. He has been right with seven of 16 in his five matches so far for a 43.8 percent success rate.

Murray has managed to get five right from 22 for a miserable 22.7 percent, with Roger Federer — who isn't too keen on the system — on three from eight. Of the men's top four, Novak Djokovic is the most accurate with an accuracy rate of exactly 50 percent.

Aussie teenager Bernard Tomic has been one of the least accurate at Melbourne Park. In his four matches, he challenged 24 times and only had the call overturned on 3 occasions.

ON A ROLL: Forget Novak Djokovic's 43-match unbeaten streak to start the 2011 season. Esther Vergeer, the top women's wheelchair player in the world, hasn't lost a singles match in nine years.

After her win in the first round of the eight-woman wheelchair event at the Australian Open on Wednesday, the 30-year-old Dutchwoman extended her winning streak to 442 matches. The last time she lost a singles match was in January 2003.

"Won my match against Annick Sevenans today. Tomorrow I will play Sabine Ellerbrock, and of course doubles starts then as well," Vergeer tweeted after the match.

Vergeer has been so dominant for so long in wheelchair tennis, she's won practically ever Grand Slam tournament she's ever entered. She's captured 19 Grand Slam singles titles, including eight Down Under, and 21 Grand Slam doubles trophies.

Vergeer, who was left paralyzed after surgery at age 8 to correct a spinal defect, is also a five-time Paralympic gold medalist.

HINGIS ADVICE: Five-time Grand Slam winner Martina Hingis has some advice for Caroline Wozniacki on how to handle the game's power hitters: Don't let them push you around.

Wozniacki, who mostly plays a defensive-minded game, fell to defending champion Kim Clijsters in the quarterfinals of the Australian Open, ensuring she'll drop from the top spot in the rankings on Monday.

The 21-year-old Dane has a losing record against many of the game's biggest hitters — she's 0-3 against Clijsters, 0-3 against Serena Williams, 0-4 against Venus Williams and 2-3 against Maria Sharapova.

"I didn't step back. I tried not to let them push me," Hingis said of playing power players like the Williams sisters and Lindsay Davenport. "They were stronger than me. It's hard to play three, four players like that in a row. It's three-setter after three-setter after three-setter.

"Today you just can't let yourself get pushed back. (Wozniacki) has to try to move in, step forward, otherwise there is always going to be somebody coming on top of her at a Grand Slam. She's a great player. I wish I would see her come in a little bit more."

Hingis, a three-time champion at the Australian Open, is playing in the legends' doubles event this week with partner Iva Majoli. It's her first time back at Melbourne Park since losing to Clijsters in the 2007 quarterfinals. She retired for a second time at the end of that season.

SILENCING CRITICS: Maria Sharapova is not going to turn down the volume for just anyone.

Eighth-ranked Agnieszka Radwanska of Poland criticized Sharapova for the shrieks she makes when she strikes the ball, describing them as "annoying" and "just too loud." Radwanska had just lost a quarterfinal to Victoria Azarenka, who also makes a loud noise when she hits the ball.

Sharapova was not amused.

When told about Radwanska's comments after her quarterfinal win over Ekaterina Makarova on Wednesday, the fourth-seeded Russian asked rather icily, "Isn't she back in Poland already?"

She added she's not about to make any changes to her playing style.

"I've been the same over the course of my career," she said. "No one important enough has told me to change or do something different. I've answered it many times before. I'm sure I'll answer it many more times ahead. I'm OK with that."

The WTA said this week it is looking at ways to deter players from incorporating grunts or screeches into their hitting motion, noting that "some fans find it bothersome."

Fellow semifinalist Azarenka has been mimicked by the crowds in Melbourne for her hooting noises while hitting the ball.

DOUBLES LIFT: Sara Errani got the perfect pick-me-up after her singles quarterfinal loss on Wednesday. She went out a couple of hours later and made her first Grand Slam final in doubles.

The 24-year-old Italian stretched second-seeded Petra Kvitova in her first major singles quarterfinal, before losing 6-4, 6-4. Then, she and fellow Italian Roberta Vinci beat seventh-seeded Czech pair Andrea Hlavackova and Lucie Hradecka 5-7, 7-5, 6-1.

"During the doubles, after the first set, I had a bit down, a bit tired," she said. "But we won a very good match. We play very good in the third set. I'm very happy for that."

The 5-foot-5 baseliner hadn't gone past the third round of a Grand Slam singles tournament before arriving in Melbourne, and a quarterfinal at the 2011 U.S. Open was the best she'd managed in doubles.

Her last match will be against Svetlana Kuznetsova and Vera Zvonareva in Friday's doubles final after the Russian pair beat sixth-seeded Sania Mirza of India and Elena Vesnina of Russia 7-6 (4), 2-6, 6-4.

Associated Press writer Justin Bergman contributed to this report.