He trails Ding Junhui 10-6 after two sessions
Five-time champion Ronnie O'Sullivan is facing elimination from the World Championships in Sheffield.
O'Sullivan only won two frames in his second session against Ding Junhui to go from 4-4 to trailing 10-6. It's the first time the two men have faced each other in The Crucible, but O'Sullivan holds a 10-2 overall record against his Chinese opponent.
Ding, the 2016 runner-up won the opening three frames in his quarter-final against O'Sullivan, before the 41-year-old battled back to end the opening session level at 4-4.
At the start of Tuesday's evening session, the duo shared the opening two frames, before Ding won five in a row, to take a crucial 10-5 lead. With his future in the World Championships on the line, O'Sullivan scored a century in the day's final frame to only trail by four frames ahead of the final session.
The duo play to a finish on Wednesday afternoon, with 13 frames needed to advance to the semi-finals.
John Higgins has become the first player to reach the semi-finals. The four-time World Champion, who last reached the semi-finals, when he won the title in 2011, defeated Kyren Wilson 13-6.
Higgins will meet either Barry Hawkins or Stephen Maguire in the semi-finals. Hawkins leads 5-3 after the opening session.