Mark Selby established a convincing 6-2 lead over fellow three-time champion Mark Williams in their world championship quarter-final on Tuesday.
The Englishman won the first two frames without conceding a point, including a break of 121, and was 3-1 ahead at the mid-session interval in a best-of-25-frame match.
Williams, whose first pot was an extraordinary five-ball plant in the third frame, reduced the deficit with a break of 60.
The Welshman led 55-6 in the first frame after the interval but couldn't prevent Selby taking it before breaks of 90 and 76 extended his lead to 6-1.
But Williams finished the session with a fine break of 111.
Meanwhile, Anthony McGill came from 6-4 down to lead 2015 champion Stuart Bingham 9-7.
The Scot, who caused the shock of the tournament so far by knocking out defending champion Ronnie O'Sullivan, made breaks of 126, 83, 92, 130 and 75 to open up a two-frame advantage.
Kyren Wilson, last year's runner-up, also fought back to draw level at 8-8 with Neil Robertson.
Australia's Robertson, the 2010 champion, led 5-3 after the first session.
Wilson recovered to 6-6 before a break of 126 saw Robertson go ahead.
The Englishman won the next two frames but Robertson took the final frame of the day with a break of 66.
World number one Judd Trump and Shaun Murphy, both former world champions, are all square at 4-4, with seven of the frames featuring breaks of 67 or higher.