Novak Djokovic fears he will miss the Australian Open after being forced to pull out of the Mubadala WTC exhibition in Abu Dhabi.
The former world No 1 has withdrawn from the tournament due to pain in his right elbow.
He will be replaced by Andy Murray who has travelled to the UAE to continue his preparations for the new season.
Djokovic’s participation in the first Grand Slam event of the year is now in doubt as he awaits scan results, with the Australian Open starting on January 15 in Melbourne.
The Serb has not played since Wimbledon in July after suffering an elbow injury.
Djokovic said on his official website: “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.
“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.”
Murray will now face Spain’s Roberto Bautista Agut in the semi-finals with Dominic Thiem and Kevin Anderson contesting the other last-four match.
The contest was supposed to be Djokovic’s first outing since he retired from his quarter-final against Tomas Berdych at Wimbledon.
He added: “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.”
Djokovic has dropped to 12th in the world rankings during his absence and he is now in danger of falling further if scans reveal any significant damage.