UK Championship: Ronnie O'Sullivan dreadful - Mark Williams
|UK Championship 2016|
|Venue: York Barbican Dates: 22 November to 4 December|
|Coverage: Watch live on BBC TV, Connected TV, Red Button, BBC Sport website and app from Saturday, 26 November.TV times and channels|
Mark Williams has described Ronnie O'Sullivan's performance in their UK Championships quarter-final as "dreadful", despite being beaten 6-2.
O'Sullivan capitalised on some sloppy play from the Welshman to reach the semi-finals at the York Barbican.
"It's probably the worst I've seen him play when I've played him and he's still hammered me 6-2," said Williams.
England's O'Sullivan will now face Hong Kong's Marco Fu, who came from 5-2 down to beat Wales' Jamie Jones.
Williams, 41, had kept the five-time world champion back to 2-2 on Friday, but O'Sullivan closed out the win by taking four frames in a row.
"I've been poor all through [the tournament] and I've just managed to scrape through," Williams, who won the UK Championship in 1999 and 2002, told BBC Radio Wales after progressing to the last eight for the first time since 2010.
"I wouldn't say he [O'Sullivan] was up to gear - but he was dreadful.
"I used to get surprised at how bad I play, but I'm not any more. I let the conditions get to me a little bit.
"You can't play that badly and just expect to get anywhere near anyone."
O'Sullivan, a UK Championship winner on five occasions, said he "had too much" for his opponent.
The 40-year-old added: "It wasn't the greatest of performances but you have to do what you have to do.
"I am sure my game will get stronger. I am a fighter and will keep fighting."