Victoria Azarenka of Belarus looks on during a match at Wimbledon in 2017. Photo: AP Photo/Tim Ireland

MELBOURNE - Former champion Victoria Azarenka, who has been locked in a custody battle over her young son, pulled out of the Australian Open on Monday.

The two-time winner had been handed a wildcard into the opening Grand Slam of the year in Melbourne, starting on January 15, but will not be making the trip.

"It’s unfortunate that Azarenka is unable to travel to Australia this year," tournament director Craig Tiley tweeted. "The Australian Open is her favourite tournament and she's looking forward to returning to Melbourne next year."

Azarenka returned from maternity leave in the middle of last year but skipped the US Open after a Los Angeles judge presiding in a custody case over her son Leo ruled she could not leave California until the matter had been settled.

She had been in doubt for Melbourne Park after pulling out of last week's Auckland Classic.

The Belarusian is the latest top name to withdraw from the Australian Open.

Serena Williams, herself a new mother, opted out last week, saying she was still not at the level needed for a major tournament after giving birth. Andy Murray and Kei Nishikori are also non-starters after succumbing to injury.

Meanwhile, Rafael Nadal and Novak Djokovic will use the Kooyong Classic starting Tuesday as a convenient emergency stop to gain much-needed match practice as the pair return from injury in time for the Australian Open.

Not since the glory days of Andre Agassi and Pete Sampras nearly two decades ago and more recently Roger Federer, has the long-time exhibition event hosted as distinguished a field.

The upcoming edition will feature five of the world's top 10, including ATP Finals runner-up and two-time Kooyong champion David Goffin, Austrian Dominic Thiem, Croat Marin Cilic and Spain's Pablo Carreno Busta.

Under the ad-hoc rules, late additions Nadal and Djokovic will be playing practice matches.

World number one Nadal, who missed the warm-up Brisbane International last week with a knee injury, is set for just one appearance on Tuesday when he faces France's Richard Gasquet.

Djokovic, who hasn't played competitively since a right elbow problem forced him to quit Wimbledon in the quarter-finals in July, is due on court on Wednesday to play Thiem.

"The addition of both Novak and Rafa changes our schedule and the way we will run the four days but that's what Kooyong is all about, we are here to help the players get ready for the Open," said tournament director Peter Johnston.

The women's draw, now into a second season after being introduced in 2017, includes new Hopman Cup champion Belinda Bencic of Switzerland, German Andrea Petkovic, Aussie teen Destanee Aiava and young Chinese star Wang Xinyu.

Pat Cash, the 1987 Australian Open winner who now coaches American CoCo Vandeweghe, serves as official ambassador for the event and said it would be good preparation for Djokovic and Nadal.

"Everyone will be watching Rafa's legs and Novak's arm," he said. "It can make for a very exciting week of tennis. Both guys had good preparation, they just suffered setbacks. 

"But when those come within a couple of weeks from the Australian Open, it's a bit of a worry. They have to be absolutely ready, players can be quite anxious about it all."

The first day of play features four matches, with Nadal against Gasquet, Goffin facing Carreno Busta and wayward Australian Bernard Tomic taking on Japan's Yoshihito Nishioka.

Among the women, young Australian Aiava plays Petkovic.

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