O'Sullivan, the former world junior champion from Essex, at 17 years and four months the second youngest player behind Stephen Hendry to appear at Sheffield's Crucible Theatre, suffered a 10-7 defeat at the hands of the Scottish player.
McManus refused to let his frustration at losing four frames on the black and another on the pink affect him. Although O'Sullivan recorded a break of 79 to draw level at 7-7, a run of 40 followed by two of 63 took him through to a second-round meeting with either the six-times world champion, Steve Davis, or Peter Ebdon.
'What I thought was good enough was nowhere good enough,' O'Sullivan said. 'Players like McManus are on a different planet so God knows how good Stephen Hendry is. My long potting was way out and you can't afford that at The Crucible. Alan is so strong in the safety department and the standard of his potting surprised me.'
McManus, beaten 16-7 by Jimmy White in the semi-finals a year ago, said: 'I was disappointed to be only 5-4 up after the first session and it did affect me. I could easily have been 7-2 ahead but I made a couple of daft mistakes. Possibly, experience told in the end. Ronnie is a fantastic player but he's only 17 and he will learn through meeting the top players.'
As for his next match, he added: 'Playing Davis at The Crucible has always been a great ambition of mine but I think Ebdon has a great chance of winning again. Whoever I meet, I will have to sweat blood.'
Dennis Taylor, the champion in 1985, defeated Tony Drago 10-9 in a compelling match to keep alive his hope of staying in the world top 16. Though Taylor was never behind, Drago came back from 8-6 down to take the match into the deciding frame. Drago had the first chance, but missed an easy black, and Taylor went through with a break of 46.Reuse content