Williams trailed McManus 5-4 at the open having been down 5-3 at one point in the first session.
How the McManus appeared to be in wretched form in stark contrast to his earlier performance, and a string of errors allowed Williams to pick up momentum.
Breaks of 68, 50, 79 and 59 were part of a seven-frame run that gave McManus little reason to hope for a way back once he fell behind.
The former World Champion said after the win that coronavirus and the ensuing lockdown had inspired him to recommit his long-term future to his career.
Snooker referee calls on VAR to check the what order the balls touched
Ding Junhui was taken to the limit by Mark King as he won 10-9 in the first round at an empty Crucible.
Ding started the session with a failure to take his chances at 5-4 down, with King fighting back to level the scores at 5-5. That doggedness would define the effort put in by the veteran.
He then hit a beak of 125, before getting 119 to show his impressive post-lockdown form, but King refused to bow down and pulled the scores to 8-7 behind.
Ding edged ahead 9-7 with a break of 67, leaving him just one frame from victory and a second round match that might see him face Ronnie O’Sullivan.
King wasn’t finished, nor was he ready to let go of the game. He edged an exhausting 17th frame with an excellent period of safety play to take it to 9-8. He levelled the tie at 9-9 with an impressive double on the pink to leave the match in the balance.
King pushed his rival all the way in the final frame which was only settled once the reds were potted and Ding seized a routine clearance of the colours to claim a hard-fought win.
John Higgins, meanwhile, took a 6-3 lead over Welshman Matthew Stevens, taking the last four frames of the session consecutively and moving into ominous form, with breaks of 60, 61, and 69 to take control of the match.
Stuart Bingham is through to the second round of after a hard-fought 10-7 victory over Ashley Carty.
With the 44-year-old starting the second day of the championships 5-4 up, but without a crowd today, the 2015 world champion began the morning session brilliantly and grabbed his 27th Crucible century 131-0 to take a two-frame lead.
Stuart Bingham 'puts on show' with century break
Despite missing a good chance to secure the frame with a 147, Bingham took a 96 break to pull further away and go 7-4 up before taking the next two 96-0 and followed up with a break of 58 in the 13th frame.
Carty fought back though, winning the next three frames in a row. At 9-5, Bingham missed a simple green allowing Carty to capitalise. The 25-year-old potted the pink to make it 9-6 with a 69-42 frame.
Carty won the next frame 68-6 before Bingham finally regained his composure to see it off 10-7 with a break of 82.
Bingham will face either Mark Williams or Alan McManus in the second round.