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."
Reigning champion Mark Selby feels his World Championship defence is warming up nicely after a second routine win secured a quarter-final place.