Williams had been in action in the second round of the World Championship on Friday afternoon against David Gilbert.
However, after the end of the opening session Williams tweeted he had gone to hospital after suffering from chest pains.
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"Doctors are confidant it’s not anything to do with my heart," said Williams in a tweet. "Awaiting more tests at 10 o clock."
The on-call doctor at the Crucible advised Williams to get the problem checked out, but he did not use an ambulance preferring to go by car.
"A&E. Could be hear [sic] a while, couldn't stick the chest pains no more," he said. "Lucky there wasn't any more frames to play."
Williams is scheduled to play in Saturday's morning, starting at 10am, and the closing evening sessions. He trails Gilbert 5-3 in their best-of-25 encounter.
Williams won the third of his world snooker titles last season, 15 years after his second success at the venue.
"The snooker family hope that everything is okay," said 2005 world champion Shaun Murphy. "We live in this little bubble where we think snooker is the be all and end all, but it obviously isn't."