MELBOURNE: Chinese tennis star Li Na said the early passing of her father taught her a significant lesson about the value of hard work that has held her in good stead in her playing career.

Writing in The Age newspaper, she also said some young professionals aren't reaching their potential because they are being spoilt by over-indulgent mentors.

The world No 11 advanced to the quarterfinals of the Australian Open yesterday, with a straight sets win over 21-year-old Belarusian Victoria Azarenka.

A semifinalist here last year, when Li and countrywoman Zheng Jie created history by becoming the first Chinese players to reach that stage of a Grand Slam, she is playing some of the best tennis of her career at the age of 29.

But Li questioned whether some of her younger rivals shared her hunger for success, which she puts down to some painful early experiences.

"My father died when I was 14," she wrote. "Some young players nowadays do not appreciate that they are given whatever they want.

"They don't even need to work hard and they still get whatever they want.

"When my father died, mum had to take care of everything, so I couldn't tell her what I wanted because it was hard for her.

"So I think I have always believed in hard work."

Li also credited her husband, Jiang Shan, with helping her become a more focussed player by being a calming influence.

"He has taught me to calm down, to think about what I do," she said. "He can understand what I do on the court."

Li also revealed that her mother refuses to watch her games because she gets too nervous, and even shunned seeing her at the 2008 Beijing Olympics.

"I asked mum to come and watch me play but she said: 'No, no, no, I don't want to go'," Li said.

"She came to Beijing but she didn't come to the court to watch me. She's not a sportsperson and she has always been nervous about watching me play."

In this morning's matches, French Open finalist Robin Soderling's eight-match streak came to a sudden halt in an upset 1-6, 6-3, 6-1, 4-6, 6-2 fourth-round loss to Alexandr Dolgopolov.

Soderling won the lead-up tournament at Brisbane without losing a set and hadn't conceded a set at Melbourne Park before he confronted the 22-year-old Ukrainian, who has reached the quarterfinals in his first trip to the Australian Open.

Dolgopolov's backhand crosscourt to bring up his first match point was typical of the 50 winners he hit against a stunned Soderling, the highest of the seeded players knocked out of the men's draw. Soderling saved three match points, but his run came to an end with another unforced error, his 51st.

The Czech Republic's Petra Kvitova downed Italy's 22nd seed Flavia Pennetta 3-6, 6-3, 6-3 in the fourth round. The 28th seeded Kvitova came back from losing the first set to beat the Italian in 1hr 55min on Melbourne Park's Hisense Arena.

Kvitova, who defeated fifth seeded Australian Samantha Stosur in the third round, has now won nine matches in a row to start the year, following her win in the Brisbane International in the first week of January.

The 20-year-old left hander from Fulnek in the Czech Republic reached the semifinals of Wimbledon in 2010, losing to eventual champion Serena Williams.

Kvitova went into the Australian Open with a career-best ranking of 28 and is now projected to break into the top 20 after reaching the last eight.

She is also the first Czech woman to reach the quarterfinals in Melbourne since Nicole Vaidisova in 2007. She will take second seeded Russian Vera Zvonareva for a place in the semifinals.

Zvonareva continued her quest for a first Grand Slam title with a commanding 6-4, 6-1 victory over unseeded Czech Iveta Benesova to reach the quarterfinals this morning.

RESULTS
This morning
Fourth Round
Men
Alexandr Dolgopolov (UKR) bt Robin Soederling (SWE) 1-6, 6-3, 6-1, 4-6, 6-2
Women
Petra Kvitova (CZE) bt Flavia Pennetta (ITA) 3-6, 6-3, 6-3; Vera Zvonareva (RUS) bt Iveta Benesova (CZE) 6-4, 6-1

Yesterday
Men
Novak Djokovic (SRB) bt Nicolas Almagro (ESP) 6-3, 6-4, 6-0; Tomas Berydch (CZE) bt Fernando Verdasco (ESP) 6-4, 6-2, 6-3; Roger Federer (SUI) bt Tommy Robredo (ESP) 6-3, 3-6, 6-3, 6-2; Stanislas Wawrinka (SUI) bt Andy Roddick (USA) 6-3, 6-4, 6-4
Women
Caroline Wozniacki (DEN) bt Anastasija Sevastova (LAT) 6-3, 6-4; Li Na (CHN) bt Victoria Azarenka (BLR) 6-3, 6-3; Francesca Schiavone (ITA) bt Svetlana Kuznetsova (RUS) 6-4, 1-6, 16-14; Andrea Petkovic (GER) bt Maria Sharapova (RUS) 6-2, 6-3

MELBOURNE: German's Andrea Petkovic had just beaten Nadia Petrova in a tough three-set match at last year's US Open when her coach reminded her about a pre-match promise to do something different if she won.

The hip-shaking wiggle that followed has become a good-luck charm she has used ever since, and that she credits with helping her reach her first Grand Slam quarterfinal at the Australian Open.

The 23-year-old became the first German to reach an Australian Open quarterfinal since Steffi Graf in 1999 by beating former No 1 Maria Sharapova 6-2, 6-3 yesterday.

The poli-sci student who defied her parents wishes to pursue a tennis career, Petkovic has also been winning over crowds in Australia with her bright smile, upbeat attitude, and the courtside dance that has become known as "The Petko."

"It started off as a bet with my coach," she said. "At the US Open I was playing terrible. I got to the first round and he said, 'OK, if you win this you have to do something special.

"I won 7-6 in the third and I wanted to flee the court. But my coach said, 'Hey, the dance.' So I did a little this," she said, demonstrating a step or two, to loud cheers from the crowd.

"It became so famous. Also, I am superstitious — I have played much better since I do the dance," she said. "So, everybody, if something is not going his way or her way, they should start dancing. It helps."

She said her father, Zoran, who played Davis Cup twice for Yugoslavia in the 1980s before moving his family to Germany, tried to talk her out of chasing a tennis career because of the pressures of the tour. He was all smiles in the stands.

Sharapova, a three-time Grand Slam winner who is trying to come back from an extended injury break in 2008-09, described yesterday's match as a bad day at the office, but said she would keep it in perspective.

"I think I can say there are a lot of more painful things in life than losing a tennis match," the Russian said.

"There's a lot more in life than hitting a tennis ball. We're all pretty fortunate to be able to do what we do, win or lose. Obviously wins bring you a lot of smiles and laughter, but this is sport."