Two days after booking passage to the Eastern Conference semifinals, the Celtics now know their opponent: the New York Knicks.

The third-seeded Knicks defeated the No. 6 seed Detroit Pistons 116-113 on Thursday to win their first-round series in six games and set up a matchup with their longtime Atlantic Division rivals.

It’s the first postseason meeting between the Celtics and Knicks since 2013, when New York eliminated Boston in the opening round to close out the Paul Pierce/Kevin Garnett era. The last time any Boston and New York teams met in the playoffs was the 2021 MLB wild-card game between the Red Sox and Yankees.

All-Star point guard Jalen Brunson scored 40 points in Game 6 against Detroit, including a go-ahead step-back 3-pointer with 5.1 seconds remaining. The final five games of that series all were decided by six points or fewer. The Knicks trailed late in Thursday’s clincher but closed the game with an 11-1 run.

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After swinging blockbuster trades for Karl-Anthony Towns and Mikal Bridges last offseason, the Knicks compiled the NBA’s fifth-best regular-season record (51-31) while boasting one of the league’s top offenses (fifth in offensive rating). They struggled mightily against the Celtics, however, dropping all four matchups by an average of 16.3 points.

Boston opened its season with a 132-109 rout over New York — tying the single-game NBA record for made threes in the process — then won 131-104 at Madison Square Garden on Feb. 8 and 118-105 at TD Garden on Feb. 23. Only the teams’ final game was close, with the Celtics winning 119-117 in overtime at MSG in the final week of the regular season.

The Celtics were without starting center Kristaps Porzingis in the first two contests, and Jaylen Brown was limited in the final encounter, playing just 22 minutes and sitting out the fourth quarter and OT. Starting guard Jrue Holiday also sat out the Feb. 8 win.

Boston head coach Joe Mazzulla downplayed the significance of the season sweep.

“I mean, it means nothing,” Mazzulla said after the OT win on April 8. “If anything, it’s more dangerous for us. … There’s so much that needs to happen before that, and if you don’t handle what you’ve done against someone else, then that can actually be more detrimental to you psychologically. So it all depends on how we continue to get better as a team.”

The defending champion Celtics earned the No. 2 seed in the Eastern Conference and won their first-round series against the Orlando Magic in five games.

Games 1 and 2 of the East semifinals will be next Monday and Wednesday at TD Garden. The Knicks will host Games 3 and 4 next Saturday and the following Monday.