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. Ronnie O'Sullivan is spoken to by referee Olivier Marteel in his second round match against Matthew Stevens . O'Sullivan is spoken to by referee Marteel in his second round match against Stevens at the Crucible . 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. Stevens reacts to a shot in his match against O'Sullivan during day nine of World Championships . Stevens in action against O'Sullivan in their second round match  at the Crucible Theatre . 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. O'Sullivan in action against Stevens in their second round match  at the Crucible Theatre in Sheffield . 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.
Ronnie O'Sullivan is closing in on World Championship quarter-final spot . But, he found himself in a spot of bother with referee Olivier Marteel . O'Sullivan was warned about his behaviour after obscene hand gesture . Five-time world champions needs one more frame to secure progression .