The 46-year-old looked in good form at York Barbican and hit two century breaks to take a step towards claiming a record eighth title.
While O'Sullivan clinched a record-equalling seventh world crown in May and backed it up with victory at the Hong Kong Masters in front of 9,000 fans last month, he revealed his flatness after a routine round-one success on Monday.
"I feel flat, I'll be honest with you," he told BBC Two.
"I don't really want to play snooker at the moment, I don't play much snooker. I just listen to my body, listen to my mind and just go out there and treat it like a practice.
"One thing I am doing is applying myself mentally. Whether I play good or bad, I don't really care and I will always apply myself well while I am out there, but I don't feel buzzed up at the moment. I suppose that is normal.
"I am just here enjoying York - it is such a lovely place. I am going to be here for the duration, no matter what. I just like it here, you know."
The world number one rejected suggestions in the summer he was the greatest in the sport's history after his Crucible triumph.
And he again raised questions about how long he will continue to play snooker following this win over Welshman Stevens, who after taking the fifth frame to reduce the deficit to 3-2 had no answer to the Rocket.
Too old for this
O'Sullivan added: "I shouldn't even be playing at my age really, by rights.
"Look at (Stephen) Hendry, (Steve) Davis and those guys, so I don't take it too seriously and just find it quite funny I am still here. I feel like I have a bit of imposter syndrome.
"I felt happier when I lost them five finals because it felt right but when I've started to win a few tournaments, I don't really feel like I deserve it or that I have played well enough.
"It feels like there has been a plot maybe to allow me to do it, so you feel like a bit of a fake.
"I feel a bit not good about it, so that is why when I won the worlds I was saying it wasn't really a great thing for me because it put me in that place again of questioning myself and the game.
"I don't get excited like I used to. I am here to do a job."
O'Sullivan's last-16 opponent will be Zhou Yuelong, who edged out Chinese compatriot Yan Bingtao 6-5.
Zhou made a strong start and produced a 128 break, the highest of the tournament so far, but had to hold his nerve to win a back-and-forth encounter in the afternoon session.
Jamie Clarke and Stuart Bingham progressed in the evening session on day three at the York Barbican.
Clarke won his all Welsh battle with Mark Williams 6-3, while four half-century breaks helped Bingham see off Liam Highfield 6-2.