De Bruyne and City have made a scintillating start to the campaign, winning seven and drawing one of their opening eight games to be two points clear at the top.
Jose Mourinho's United are the only other unbeaten side remaining and they hold a three-point buffer to third-placed Tottenham.
De Bruyne said it appeared the title battle was between City and United – for now.
"For the moment I think so, but everybody's still there," the Belgium international told ESPN FC .
"It's so early, if you lose two or three games you go back to a [lower] position. We have to maintain this hopefully."
Pep Guardiola's side have been lauded for their style this season and have netted 29 league goals and conceded just four.
De Bruyne believes City are producing the types of performances their Spaniard boss wants.
"I think we're pretty much what he wants," he said.
"Obviously one game is going [to be] better than the other ones and one time we will look more like his team than another, but I think the way we play is always the same.
"We play dominant football and like to put a lot of pressure on them and if you can do that, it works well. It's hard to go full pace all the time but at the moment everyone is in great shape."