Ronnie O'Sullivan trails Ali Carter 9-7 in their Betfred World Championship second-round match.
O'Sullivan, who beat Carter in the final in both 2008 and 2012, is seeking a sixth Crucible title.
However, he went into the second session of their best-of-25-frame contest 5-3 down and his chances of progress were not helped as he lost the first three frames.
Although O'Sullivan ended the poor run by winning the next two, he was far from his fluent best and Carter made him pay with a break of 51 to go 9-5 ahead.
However, O'Sullivan finished with a flourish, as breaks of 79 and 105 saw him reduce the deficit to two frames again. The pair resume on Saturday afternoon.
Mark Allen became the first player to book his place in the quarter-finals with a 13-8 victory over Joe Perry.
The pair resumed at 8-8 after Perry - first-round conqueror of defending champion Mark Selby - overturned a 5-3 deficit.
However, Allen took control of the contest by winning the first four frames of the evening, with breaks of 133 and 74 helping his cause, before wrapping victory up with a clearance of 122.
Earlier, Lyu Haotian hit back from an 8-3 deficit to trail Barry Hawkins 9-7 in their best-of-25-frame encounter, while Kyren Wilson established a 5-3 advantage over Jamie Jones. Both matches are due to resume on Saturday morning.