The world number 18 is down 10-6 against O’Sullivan, and the match is being played to the end on Monday afternoon at 13:00 BST.
Having secured a 5-3 advantage from Sunday’s morning play, English-seasoned player O’Sullivan went on to win five of the first six frames in the evening session, aided by noteworthy breaks of 82, 65, 89, 56, and 96.
However, Day, who had defeated 15th seed Barry Hawkins in the first round, closed off the match with decisive breaks of 75 and 72 to win the final two sets.
Nevertheless, O’Sullivan will be a heavy favorite to make it to the final eight, when he will face either 2015 Crucible winner Stuart Bingham or world number 17 Jack Lisowski.
The 48-year-old O’Sullivan wants to win an eighth global title to go one better than Stephen Hendry.
