Ronnie O'Sullivan eclipsed the highest break of this year's world championship yesterday but it was not enough to help him to a first-session lead against Ryan Day.
"The Rocket" beat the 139 he made in his 10-4 victory over Dave Harold by knocking in a 140-total clearance in frame six, which levelled the match at 3-3 but with Day far from overawed by his task.
A 10-3 thrashing of Joe Perry in the opening round suggested the Welshman had the credentials to test O'Sullivan and that looks like being borne out as the session ended 4-4.
The world No 1 began well, taking the first two frames without needing to make a half-century, but Day hit back by clinching the next three.
In the frame before the interval, O'Sullivan conceded and left the arena while his opponent was still at the table - an act considered unsporting but it did not incur an official warning.
After the two-time former champion had compiled his 140, Day produced a 68 to edge back ahead before a 65 from O'Sullivan ensured parity ahead of the next stint set for tomorrow morning.
On the other table progress was slower, but unsurprisingly that did not affect Peter Ebdon as he built a 4-1 lead over David Gray, first-round conqueror of Jimmy White.
On Saturday, two-times former champion Mark Williams joined the title-holder Shaun Murphy in the quarter-finals.
Welshman Williams, who completed a 13-8 victory over England's Mark Selby, could face O'Sullivan in the last eight. However, the 31-year-old is not daunted by the prospect. "If you win the world championship, you want to play the best and beat the best and he is the best," Williams said.
Defending champion Murphy eased through with a 13-7 win over six-times former champion Steve Davis. Murphy, who says he is "90 per cent there" after battling to shake off a virus. "Sooner or later I'll find my potting boots and when I do, the rest of the players will be in trouble," said the 23-year-old Englishman.
Qualifier Marco Fu of Hong Kong beat the world No 3 Stephen Maguire of Scotland 13-4 to reach the last eight. Fu said he has been inspired by the impressive progressive made by China's Ding Junhui, saying: "I hope I can do as well as Ding because he was a big inspiration to me when he won the UK."
Ding captured his first main tour event in Beijing last year and followed that up by winning the 2005 UK championship.