O’Sullivan raced into a 3-0 lead, with the help of breaks of 115 and 79, but Williams dug in with a vital 55 to win the fourth frame.
A break of 83 reduced O’Sullivan’s lead to 3-2, but the seven-time champion claimed the next and still looked a good favourite. But Williams then made a tournament-high break of 143 and a 90 to level at 4-4.
Williams took the lead for the first time with a gutsy 59, but O’Sullivan fired in a 77 to force a decider. The Welshman compiled a cool 102 for a victory he delighted in, with the Alexandra Palace crowds as raucous as ever.
‘The first three frames I didn’t have a shot,’ Williams said. ‘He tied me up in knots, making breaks galore.
‘But I felt the crowd started to sway towards me at the end and most of them wanted me to win, which was unbelievable. I don’t think I’ve beaten him in the UK for 20 years.
‘That’s gone now. If I don’t beat him for another 20 years I couldn’t care.’
O’Sullivan said: ‘I just didn’t score. There was far too many mistakes.’
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