Ronnie O'Sullivan found himself in more bother at the Betfred World Championship after making a rude gesture during his second-round match against Matthew Stevens.

On the table the five-time world champion enjoyed a successful day, firing two rapid centuries in establishing a 12-4 lead, needing just one more in Monday’s closing session to the match to reach the quarter-finals.

But O’Sullivan was also spoken to by the referee Olivier Marteel towards the end of the afternoon session — and warned about his behaviour.

Marteel approached O’Sullivan as he walked to the table when Stevens missed a red, telling O’Sullivan he had noticed his hand gesture, which was thought to have been self-directed.

O’Sullivan avoided a fine in the first round when he briefly played without shoes after a new pair proved uncomfortable, breaking the rules but not sufficiently to warrant any punishment, after he accepted a substitute pair of size eights, loaned by tournament director Mike Ganley.

The warning from Marteel could be the end of Sunday’s matter. Yet any repeat of the incident would see O’Sullivan docked a frame, a tournament spokesman confirmed.

Armed with a 5-3 lead from the opening session, O’Sullivan took the opening two frames on Sunday to pull further in front.

The next went against the 39-year-old, but breaks of 110 and 139 in consecutive frames suggested O’Sullivan was in unstoppable form, as sweeping through the subsequent three testified.

Ali Carter continued to struggle in his match against Australia's Neil Robertson.

The 35-year-old from Essex, who trailed 6-2 following Saturday's opening session, won the first frame of the day but lost the next five.

A finish with a session to spare was possible when Robertson led 11-3, but Carter took the last two frames of the day to avoid that indignity and trail 11-5 overnight.

Earlier, Shaun Murphy left Joe Perry with a mountain to climb as their second-round clash got under way, surging 7-1 in front, while Judd Trump built an early 5-3 lead over Marco Fu.