Injured Nishikori out of Australian Open

The Asian number one has not played competitively since last August after suffering a torn tendon in his right wrist during a practice session at Cincinnati.

"I am very sorry to announce that I will miss the Aussie Open this year," he said on his website of the opening Grand Slam of the year, starting in Melbourne on January 15.

"The Aussie Open is my favourite Slam ... it's my 'home' Slam.

"It's the Slam that takes incredible care of the players and their teams, and it hurts that I will have to miss it this year.

"My rehab is going well but I am just not ready 100 percent to come back yet in best of five set matches."

Nishikori is not the only top player struggling with injury, with a who's who of the top names in the game battling to be fit for Melbourne Park.

Former world number ones Novak Djokovic and Andy Murray are both in doubt for the tournament.

Twelve-time Grand Slam winner Djokovic hasn't played since a right elbow problem forced him to quit Wimbledon in the quarter-finals in July.

He has already cancelled scheduled appearances at an exhibition in Abu Dhabi and the Qatar Open, but said late Wednesday he will test his elbow next week at the Kooyong Classic in Melbourne and a Tie Break Tens tournament in the city.

Murray is also struggling, withdrawing from the ongoing Brisbane International as he works to recover from a right hip injury which has kept him out of action since Wimbledon.

The writing was on the wall for Nishikori, who also withdrew from the Brisbane event.

The 28-year-old, who reached a career high world number four in 2015, said in November he had chosen rehabilitation over surgery to repair the damage and was targeting a return to action at Brisbane.

It is a major blow for the popular Japanese player, who concluded his 2017 season with a 30-13 match record.

The winner of 11 ATP Tour titles, he made the fourth round of the Australian Open last year, losing in five sets to eventual champion Roger Federer.