LONDON - Defending champion Ronnie O'Sullivan has withdrawn from the Snooker Masters on medical grounds, the World Snooker Tour revealed on Friday.

The annual invitation tournament featuring the Tour's top 16 players opens on Sunday at Alexandra Palace in London. O'Sullivan, 49, was due to face John Higgins in the tournament's opening match.

O'Sullivan's withdrawal comes after he exited a Championship League event in controversial circumstances. During a defeat to Robert Milkins, O'Sullivan angrily struck his cue against the table after failing to make an easy pot.

The Briton's place in the Masters will be taken by Australia's Neil Robertson, who was ranked in 17th position at the seeding cutoff after the UK Championship.

READ MORE: O'Sullivan ousted from Snooker World Championship quarterfinals

China's Ding Junhui was drawn to play against veteran Mark Williams in the first round on Monday. The tournament's two other Chinese players, Si Jiahui and Zhang Anda, begin their campaigns on Tuesday and Wednesday respectively, with Si facing Mark Allen, and Zhang locking horns with Kyren Wilson.