English Open: Ronnie O'Sullivan could 'pull out' of tournament
Five-time world champion Ronnie O'Sullivan complained of a sprained ankle and said he could "pull out" of the English Open in Barnsley.
O'Sullivan beat Zhang Anda 4-1 to reach the second round but - having worn trainers - has been told by organisers he must wear the correct footwear.
World number one Mark Selby won 4-3 in a final-frame decider against Scotland's Scott Donaldson.
Judd Trump defeated Robbie Williams by the same scoreline.
At the 2015 World Championship, O'Sullivan escaped a fine for playing in just his socks against Craig Steadman as his shoes caused him discomfort.
The year before at the UK Championship, the 41-year-old went into the tournament with a broken ankle, which caused him discomfort in the early stages, but still went on to win the title.
O'Sullivan, who faces Mark Davis in the next round, said on Twitter: "Being told by World Snooker that if I don't wear a black training shoe on Wednesday, I can't play.
"Looks like I will have to pull out of the English Open because someone's not happy I am wearing a trainer due to a sprained ankle."
He added: "I am just treating it like a holiday. I wandered up to Sheffield, filled my fridge up with lots of food and will be cooking on my boat tomorrow.
"I pitch up, hit a few balls and see how it goes. I have nothing to lose and everything to gain."
The English Open is the first of four Home Nations events, with a £1m bonus on offer to the player who wins all four.