The Jester from Leicester toppled the six-time Crucible king in the Scottish Open final last month and is ramping up preparations for a crack at The Masters next week.
Selby, the current world No 4, is a 19-time ranking event winner and celebrates his 38th birthday this year while world No 2 Neil Robertson turns 39, and O’Sullivan believes both players need to cash in now.
“Maybe Selby has only got four good years left in him,” he said.
“This is a big time for Neil and Selby because I think they’ve got four or five years, and then it’s inevitable that they’re going to taper off. They’re in their prime, so they’ll want to make the most of the next four or five years to fill that trophy cabinet up.
“At 37, I felt as good as I’d ever done and at 38, even at 40, I felt great. (Also at) 41 or 42, but I’ve got to be honest, the last 18 months I just can’t recover physically from one tournament to the next.”
“I just don’t, and I just get mentally tired – and I’m quite strong mentally. You can say you can still do it, but the body doesn’t allow you to.
“It used to be 33 or 34 for a snooker player but I think now it’s maybe early 40s [the end of your peak]. After that, you’re definitely on a decline.
“I don’t care who you are, once you hit 42 or 43 there’s only one way for you, and that’s down.”
Selby soared to European Masters and Scottish Open glory this season and will take on troubled Scot Maguire on Tuesday.
The city player is a three-time Masters champion in addition to his hat-trick of Crucible crowns and UK Championship – snooker’s three Triple Crown events – double.
Selby and O’Sullivan have enjoyed a fiery baize rivalry since their first encounter in 2002 with the Rocket, a record seven-time Masters champion, winning on 19 of the 31 occasions they’ve met.
Selby had endured a barren couple of years on the snooker circuit but joining forces with top coach Chris Henry has resurrected his career.
He reached the World Championship semi-finals last summer – where he lost a 17-16 thriller against O’Sullivan – and has reached two semi-finals, in addition to those two trophies, this season.
Selby went down against world No.14 Jack Lisowski in the last four of the World Grand Prix and the Rocket believes he needs to up his consistency if he is to scoop more silverware.
O’Sullivan, who will face three-time UK champion and 2011 Masters winner Ding Junhui on Wednesday, added: “[Mark’s] worked hard at his game, he wants it and he’s trying to work with Chris Henry to make himself a better player.
“But you’ve got to do it from start to finish. Although he’s won two tournaments, that’s great, [but] you can still see Jack [Lisowski] has beaten him.
“Neil [Robertson] also found the key to the lock to play against Selby. It’s hard to keep winning.”
Watch the London Masters live on Eurosport and Eurosport app from Jan 10.
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