Defending world champion Mark Williams has revealed doctors are confident his health scare is not related to his heart after going to hospital suffering from chest pains.

The Welshman lost the opening session of his second-round match with David Gilbert 5-3 at the World Championship in Sheffield.

Later, Williams posted on Twitter that he was in the A&E department of a local hospital, saying: "Couldn't stick the chest pains no more."

The 44-year-old informed tournament officials of the issue after the session and was advised by the on-call doctor to attend hospital. Williams made his own way there and is understood to be with his wife Joanne and son Kian.

Breaks of 105, 66 and 58 had earlier helped Williams get back on level terms against Gilbert, but the latter took the last two frames of the day to carve out a 5-3 lead.

Their last-16 match is due to resume at 10am on Saturday at the Crucible.

Williams posted an encouraging update during Friday evening.

"Doctors are confidant it's not anything to do with my heart," he tweeted. "Awaiting more tests at 10 o clock . #cantwaitformebed."

On the other table, three-time champion Mark Selby compiled breaks of 64, 84 and 120 to draw level at 6-6 against qualifier Gary Wilson, who had led 5-3 overnight.

Selby edged in front for the first time with a break of 72 in the 13th frame, but Wilson responded superbly with scores of 97, 115 and 78 to take a 9-7 lead into the final session on Saturday morning.