Selby beat Xiao Guodong 13-6 in the second round to set up a meeting with either Marco Fu or Neil Robertson.
The Leicester man, who beat Fergal O'Brien 10-2 in the first round, told BBC Sport: "I don't feel as though I have peaked, but that's a good thing.
"If you peak in the first or second round the only way you can go is down."
Selby won the first three frames of his final session against Xiao to progress.
He has only managed one century so far, but has looked comfortable and clinical after a consistent season which has brought him four ranking titles - including the UK Championship.
Selby, aiming for a third world title, said his good form meant he is not being troubled by the weight of expectation that comes with being the current champion.
"I don't feel under pressure as much, but I suppose that's because of the season I have had," the 33-year-old added.
"If I had come here not having won anything and struggling for form, it would have been a bit different with the defending champion tag on my back.
"But that's not the case. I am relaxed, enjoying my snooker and it seems to be showing."
Selby begins his last-eight match on Tuesday afternoon.
"I feel like my game has not been fantastic," he said. "But it's not been poor. I am happy with that going into the quarter-finals.
"My opponent will know that when they play against me they will be in for a battle. I never give in and that has always been my philosophy."

The vessel's engine failed a mile north east of Muck Island on Thursday evening.
Weather conditions at the time were described as good with a Force 3 wind and a slight swell running.
A volunteer crew from the Larne RNLI launched two lifeboats at about 18:30 BST.
The lifeboat towed the yacht to Carrick Marina.
Larne RNLI Deputy Launching Authority Philip Ford-Hutchinson said the call-out was "a great example of both our inshore and all-weather lifeboats working well together to assist the four people onboard the casualty vessel".
Reigning champion Mark Selby feels his World Championship defence is warming up nicely after a second routine win secured a quarter-final place.
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Four people have been rescued after their yacht got into difficulty off the County Antrim coast.