Novak Djokovic's Australian Open participation is in doubt after he was forced to withdraw from an exhibition event in Abu Dhabi with an elbow injury.
The Serbian has been out for almost six months and had been due to make his return to action at the World Tennis Championship.
However, he has now pulled out of a scheduled match against Roberto Bautista Agut and admits the injury "might affect the start of the season", with the Australian Open starting on January 15.
Andy Murray, who had been at the event just with the intention of training as he recovers from his own injury problems, will now step in to face Bautista Agut, according to a post on the tournament's Facebook page.
Djokovic said in a statement on his official personal website:
"I am terribly disappointed that I am forced to withdraw from the Mubadala World Tennis Championship.
"Unfortunately, in the past few days I started to feel pain in the elbow and after several tests, my medical team has advised me not to risk anything, to withdraw from the tournament and to immediately continue with the therapies.
"I am very sad because I was eager to return to playing official matches. I enjoyed the practices and everything I did to get ready for the start of the season, including the tournament in Abu Dhabi, where I always enjoy playing.
"Now I need to accept this situation, and to wait for the results of the therapies, in order to start playing tennis again and getting back to full rhythm.
"This might affect the start of the season and the tournament plan, but the decision will be made in the following days."