The ultimate game of NBA chicken is about to start.

The Celtics begin the playoffs with a first-round game in New York tomorrow, and Doc Rivers already knows who is going to advance to the second round.

That would be the team with the greatest will. The Knicks took the regular-season series, 3-1, but as far as the Celtics coach is concerned, the same factor determined each game.

“We were the aggressor in that game, and the Knicks took it to us in the other three,” Rivers said of the difference between the Celtics’ Jan. 7 win in Madison Square Garden — the infamous Honey Nut Cheerios game — and what followed.

Carmelo Anthony lost his composure with the help of some nasty talk from Kevin Garnett, and the Celtics as a group fell apart in each subsequent meeting.

Poise, or lack thereof, is magnified in the postseason. Rivers, after the most inconsistent season of the Garnett era, honestly isn’t sure which category claims his team.

“They played with a far higher purpose than we did,” Rivers said of those three subsequent losses. “I never thought in the other three we matched their intensity, and in the first game I didn’t think they matched ours. And they lost their composure.