The match between the two former Masters winners was closer than it sounded with Selby winning the second frame and the fifth on black balls.
Elsewhere, Selby compiled 85 to pick up the third frame with a run of 56 sealing his progress in the sixth frame of the day.
McManus contributed 41 to pick up the first frame, but a 63 was not enough to see him reduce the deficit to 3-2 behind as Selby pinched the fifth frame on a re-spotted black with a fine long pot, a moment that probably buried his opponent's prospects of winning.
"The second frame was a turning point because I could have gone 2-0 down," said Selby. "I won 5-1, but it could have been a lot closer. Alan's been in a bit of form this season and I'm chuffed for him because he's a great guy.
"I got to the semis here a few years ago and lost 6-5 to Mark Allen, so hopefully I can go further this time. I've still got a bit of a cold, but I feel better than yesterday."
Selby will face Marco Fu in the first of the best-of-six frame semi-finals on Saturday after the 2007 Grand Prix winner completed a 5-3 win over Mark Joyce in the other quarter-final.
Fu continued the scoring form that saw him usurp world number one Neil Robertson in the last 16 with knocks of 71, 104, 89 and 57 good enough to edge past Joyce, who contributed 77, 108 and 59 in losing what was only his second quarter-final appearance in a ranking event.
Shaun Murphy fluked the final black in the final frame to complete a 5-4 win over Graeme Dott, who had recovered from trailing 3-0 to lead 4-3.
A 70 and 126 helped Murphy open up a three-frame advantage, but the match went the distance with his opponent compiling 64, 64 and 69 in response.
Murphy will face defending champion Mark Allen in the last four after the Northern Irishman overcome John Higgins 5-3.
Allen always looked the likelier player against four-times world champion Higgins in contributing knocks of 119, 111, 62, 54 and 80 to maintain his bid for a third straight World Open title.
World Open quarter-finals
Order of play: Friday, March 14 (best of nine frames)
Mark Joyce (Eng) 3-5 Marco Fu (HK)
Mark Selby (Eng) 5-1 Alan McManus (Sco)
Mark Allen (NI) 5-3 John Higgins (Sco)
Graeme Dott (Sco) 4-5 Shaun Murphy (Sco)
Order of play: Saturday, March 15 (best of 11 frames)
Mark Selby (Eng) v Marco Fu (HK)
Mark Allen (NI) v Shaun Murphy (Eng)
- Sports & Recreation
- Mark Selby
- Marco Fu
- Shaun Murphy
- Mark Joyce