PERTH, AUSTRALIA (AFP) - As the new tennis season gets under way this weekend, several top players are in flux with questions over their form and fitness. AFP Sport looks at the varying fortunes of seven big names.

1) Serena Williams

The former world No. 1 is attempting one of the great comebacks in sport as she returns to the court just four months after becoming a mother.

Williams, who welcomed her first child, Alexis Olympia, in September, will play an exhibition match in Abu Dhabi on Saturday (Dec 30) against French Open champion Jelena Ostapenko.

The American, who hasn't played since she won the Australian Open 11 months ago - while pregnant - is expected to defend her title in Melbourne next month.

The 36-year-old has won an Open-era record of 23 Grand Slam singles titles, and now stands to equal Margaret Court's all-time mark of 24.

She will take heart from the example of Belgium's Kim Clijsters, who retired for two years and started a family before winning the 2009 US Open, in what was dubbed the "mother of all comebacks".

2) Maria Sharapova

The Russian will look for a strong start to her season at the Shenzhen Open after a mixed return to the circuit in April following a 15-month drugs ban.

The five-time Major winner, one of the highest earners in women's sport, suffered a string of early defeats before she broke through to win the Tianjin Open in October - her first title since 2015.

Next month, Sharapova will play her first Australian Open since 2016 after improving her world ranking to 59th.

Her best Grand Slam performance of 2017 was reaching the last 16 at the US Open.

3) Rafael Nadal

Defying all expectations, Nadal brilliantly revived his glory days in 2017, winning his 10th French Open and third US Open title, and finishing the season as world No. 1.

But familiar injury doubts have made for an uncertain start to the new campaign as Nadal, 31, pulled out of the Brisbane International saying he was "not ready".

Reports have cast doubt over Nadal's recovery from a right knee injury after he earlier withdrew from an exhibition appearance in Abu Dhabi.

However, Nadal has said he will play the Australian Open and he has made a habit of overcoming fitness problems throughout his long career.

4) Roger Federer

The 36-year-old also rolled back the years in 2017 and is showing no signs of slowing down as he builds towards his Australian Open title defence.

Federer has said he's fit and ready after a "great" off-season as he returns to action at the Hopman Cup in Perth, starting on Saturday.

Much will depend on the health of his "Big Four" rivals, but Federer could continue to set new standards for longevity this year.

5) Novak Djokovic

Can the "Djoker" regain his smile? The Serb ruled tennis for nearly two years until a sudden loss of form, coinciding with speculation over his private life, in mid-2016.

The 12-time Major winner hasn't played since retiring from the Wimbledon quarter-finals with an elbow injury in July, and he shapes up as an unknown quantity heading into the new season.

Djokovic, who has Andre Agassi in his coaching team, had to pull out of his first hit-out against Spain's Roberto Bautista Agut in Abu Dhabi on Friday with a longstanding elbow injury.

6) Andy Murray

Fitness doubts linger over the British former world No. 1, who has also been off the circuit since July after suffering a hip injury.

Murray, 30, began last season on top of the rankings, but he has far more modest goals heading into 2018.

"I want to get back to playing tennis, I want to be fit and healthy and that is what is driving me just now," he told Sky Sports this week.

"I was pretty unhealthy for most of this year and I am getting there but it is a slow process," he added.

7) Victoria Azarenka

Azarenka's return from maternity leave this year was complicated by a custody battle over her one-year-old, which forced her to miss much of the season.

With her ranking down at 210th, the Belarusian former world No. 1 has been handed a wild card to play the Australian Open, a tournament she has won twice.

However, as the legal case drags on, Azarenka announced her withdrawal from next month's Auckland Classic for personal reasons, casting doubt over her participation in Melbourne.