The 1997 world champion claimed O'Sullivan is the favourite to win this year's World Snooker Championship for the seventh time.
The Rocket hasn't gone out of second gear so far in the tournament, recording comfortable wins over Dave Gilbert, Mark Allen and Stephen Maguire.
He will now play either John Higgins or Jack Lisowski in the semi-finals and should O'Sullivan win the competition, he'll equal Stephen Hendry's record in Sheffield.
Doherty believes the best is yet to come from the world number one this week.
"He hasn't really been tested. I don't think he's played at his very best yet and that's a good thing - it's a good sign," Doherty told Planet Sport.
"But he hasn't been tested either which is good which means he has a lot more in the tank and I think whoever he plays in the semi-final he's the man to beat now.
"The fact that Robertson has gone out - Robertson had better sort of form going into this but Ronnie was probably just behind him and John Higgins maybe. I think he's going to be very tough to beat now."
Asked if Hendry's record will be playing on O'Sullivan's mind as he prepares for his next match, Doherty added: "Absolutely. He'll say it's not but I think deep down it's the only record he doesn't have - seven world championships.
"To be honest I think it would be really fitting if he did do it. He probably deserves it. I think it'll be amazing."
O'Sullivan holds the majority of records outside of world title wins, including the most ranking titles won (38).
He is also the first and only person to have made more than 1,000 breaks in competitive snooker matches. O'Sullivan - a six-time UK Championship winner - also overtook Hendry's record of most wins at the Crucible - 72.
Regardless of whether he wins a seventh world title, Doherty believes O'Sullivan already has his status as the greatest player to have picked up a cue.
"I think he's already the greatest but it will certainly cement it for sure (if he wins this year's tournament)."
"We've been very lucky with these players (O'Sullivan, John Higgins and Mark Williams).
"They've been inspiring each other for so long and are still at the top of their game. We could have three of the class of 92 in the semi-finals - that would be quite incredible and Stuart Bingham who's only a couple of years after them."