In the first of the last-16 matches, 2005 winner Shaun Murphy moved into a dominant 6-2 lead over China’s Xiao Guodong.
Murphy scraped through 10-9 against Fan Zhengyi in the first round, calling his match-winning break of 50 in the decider the best break he had ever made at the Crucible after he had trailed 53-17.
But the 43-year-old Englishman found this session to be calmer as he made breaks of 79, 103, 63 and 64 to go 5-0 ahead.
World number nine Xiao took the next two frames, but Murphy ended the session well and took the last with a run of 61 to have a four-frame lead in the first-to-13 match.
That match resumes on Friday at 10:00 BST and Murphy could win it with a session to spare if he wins seven of the eight frames in that session. The third session, if needed, will take place from 19:00.
Northern Ireland’s Mark Allen holds a 5-3 lead against England’s Kyren Wilson, the 2024 champion.
Two-time semi-finalist Allen made breaks of 50 and 78 to race into a 5-0 lead, but Wilson won the last three frames of the session, helped by runs of 75 and 50.
The second session is on Friday from 14:30, before it is played to a finish on Saturday morning.