Seven-time world champion Ronnie O'Sullivan made a strong start to his last-16 match against Wales' Ryan Day and holds a 5-3 advantage.
The 48-year-old Englishman is on course for an eighth Crucible title, which would move him clear of Stephen Hendry's mark of seven successes.
Day is aiming to get into the World Championship quarter-finals for a fourth time, after losing in that round in 2008, 2009 and 2012.
He has played O'Sullivan here before, losing 13-10 having been 9-7 ahead in their last-16 tie in 2006, with Day admitting that loss had hurt.
"This place gives you lots of great memories and lots of scars," said Day on Wednesday. "That was certainly one of the scars. It's not physical on your skin, but it is lodged away somewhere deep in your mind."
O'Sullivan, who thrashed 22-year-old Welsh qualifier Jackson Page 10-1 in the first round, made breaks of 123 and 84 in the opening two frames, although world number 18 Day took the next two frames.
Breaks of 83 and 51 helped O'Sullivan go 4-2 in front, Day responded with a break of 115, but O'Sullivan restored his two-frame advantage with a 92 in the last frame of the session.
The match is the best of 25 frames, so first to 13, and will resume at 19:00 BST in the evening session before being completed on Monday from 14:30.
Scotland's Stephen Maguire moved into the quarter-final thanks to a 13-9 victory over 2005 world champion Shaun Murphy.
A fired-up Maguire had built a 10-6 lead on Saturday.
Murphy had earlier edged two frames on the black and responded with a fist pump, before Maguire did likewise, punching the table and then punching the air as he moved into a strong advantage going into the third session.
The pair have been rivals since they were children, with Maguire the leading Scottish youth player and Murphy the top English young player.
They made headlines in a so-called 'Chalkgate' incident at the 2004 Grand Prix when Maguire forgot his chalk, the first-round tie was delayed and he was docked a frame.
Maguire blamed Murphy, who had spoken to the referee, for being docked a frame, although the Englishman has always insisted he was not to blame.
Breaks of 68 and 73 from Maguire helped him win two of the opening three frames on Sunday to move one away from victory.
Murphy, the eighth seed, took the next two, but Maguire made a superb break of 127 to seal victory.
It means 11 of the 16 seeds have been eliminated in the opening two rounds.