Ronnie O'Sullivan has withdrawn from The Masters due to medical reasons.

The defending champion had been due to face John Higgins on Sunday afternoon, but now won't compete at the event in London.

Neil Robertson has been named as The Rocket's replacement and will feature in the tournament's opening match against Higgins.

Confirming the news via a statement, the WST said: "Neil Robertson has replaced Ronnie O'Sullivan in the draw for the upcoming Johnstone's Paint Masters.

"Last year's champion O'Sullivan was due to face John Higgins in the opening match on Sunday afternoon, but has withdrawn on medical grounds.

"Higgins will now face Robertson, who was ranked in 17th position at the seeding cutoff after the Victorian Plumbing UK Championship.

"The Johnstone's Paint Masters runs from January 12th to 19th at Alexandra Palace in London."

It comes just one day after O'Sullivan pulled out of a Championship League event following a mid-game outburst, with speculation surrounding his future in the sport beginning to grow.

O'Sullivan had been involved in a match against Robert Milkins when he slammed his cue against the table after missing a simple pot.

Describing the incident, commentator Dave Hendon said: "That was a concern [that O'Sullivan had an easy pot] but it's not happened. The cue has taken a bit of punishment there.

"He's apologising now to the referee and Rob Milkins. Just a rush of blood [to the head]."

O'Sullivan went on to lose the first-to-three game 3-2, but it wasn't before another mini-headloss, this time appearing to completely give up on the frame altogether.

During a shot in the fifth and final frame, O'Sullivan barely took any notice to where he was firing the cue ball into and walked away from the table before the balls had even stopped moving.

On his lax shot, Hendon added: "Still anyone's frame though... well, he's [O'Sullivan] literally just whacked them. He's given up I think."

Before later confirming his withdrawal from the tournament: "Ronnie O’Sullivan we saw earlier was clearly getting frustrated against Robert Milkins. He has made the decision to withdraw from the tournament so he won’t be playing his last match against Ali Carter."

The Masters, without O'Sullivan, gets underway on Sunday afternoon. World No.1 Judd Trump enters the competition as the outright favourite while reigning world champion Kyren Wilson has been bumped up to second favourite following O'Sullivan's withdrawal.