Judd Trump is at risk of elimination from the Betfred World Championship after falling 6-3 behind against Liang Wenbo in the opening session at The Crucible.
The former finalist, the second favourite for this year's title in Sheffield, watched as his Chinese opponent secured two centuries - 117 and 107 - while he struggled.
Trump secured the following two frames, a 58 and an 86 - before Liang responded by winning four of the next five, and demonstrated fine composure by winning three of those on colours as Trump remained competitive.
Should Liang progress beyond the first round on Thursday, it will represent the first time he has done so since 2008, when on his first appearance he reached the quarter-finals.
Mark Allen earlier eased into the second round with a convincing 10-3 defeat of Crucible debutant Mitchell Mann, finishing off a match that began on Tuesday.
Ding Junhui won four frames in a row to come back and beat Martin Gould 10-8 in his opening match and will meet either Trump or Wenbo in the second round.
Ding's was a remarkable recovery. Having led 5-4 from the opening session, he watched Gould win four of their five opening frames of Wednesday evening's session to fall 8-6 behind.
The Englishman's considerable momentum was overcome, however, when Ding impressively secured breaks of 132, 86 and 100 from the following three frames.
In the tense 18th frame, he won 59-48 to secure his progress to the second round.
"At 8-6 down I just said 'Go for it'," China's Ding, who after a run of poor form overcame three qualifying rounds to reach the main draw, told the BBC.
Gould said: "At 4-1 down I let my arm go but it wasn't meant to be. Ding played brilliantly to win the last four."
Kyren Wilson won the final three frames of the opening session to build a 5-4 lead over Joe Perry in their first-round match.
Should he succeed when they resume on Thursday, it will be the first time he has reached the tournament's second round.
Veteran Perry won three of the opening four frames while recording breaks of 87, 63 and 80.
Wilson's response came in the form of a 130 in frame three, and he followed the mid-session interval with further breaks of 58, 89 and 61 to give him a promising advantage ahead of Thursday's second session.