Kristaps Porzingis serves as a harsh reminder to the Knicks of what they’ve lost.

 It was everything the Garden had hoped for — but this time in green and white.

A decade after being hailed as the future of the Knicks, Kristaps Porzingis delivered one of his best performances yet, inflicting the most painful loss of the season on his former team. Porzingis scored 34 points (8-of-13 from 3-point range), including the game-winning shot with 41.2 seconds left in overtime, as the Celtics triumphed 119-117 at Madison Square Garden on Tuesday night.

“Playing here is always really fun,” Porzingis said. “The fans are so into the game. They love the Knicks. I enjoyed playing here as a home player, and now I enjoy it even more as the opponent.”

The former fan favorite had a slow start, scoring just two points in the first quarter. But he quickly caught fire, draining a series of deep 3-pointers, capitalizing on the wide-open looks the Knicks offered him.

Despite his injury history, which has kept him to just 40 games this season, the 7-foot-2 Latvian embraced his "unicorn" status — the term Kevin Durant coined for players with boundless potential and unique skills. Porzingis, who made his lone All-Star appearance with the Knicks in 2018, was every bit the player the Celtics needed.

Leading Boston back from an early 11-point deficit, Porzingis took over in the third quarter, knocking down four 3-pointers, including a jaw-dropping 39-footer from the logo that left the Garden crowd stunned.

“The ball rolled to me, and I thought, ‘This is destiny,’” Porzingis said. “I just let it fly without thinking too much. Those are the best nights — when you’re playing free, not overthinking, and the shots are going in. Once I saw the ball keep going in, I just kept letting it fly. It was a good night.”

Porzingis took a step back in the fourth quarter as Jayson Tatum took the reins for the Celtics. But in overtime, the Latvian big man came through with a pair of clutch free throws to give Boston a crucial lead with 1:51 remaining.

With the game tied at 112, Porzingis stepped up once more. As Karl-Anthony Towns helped on a driving Derrick White, Porzingis calmly stepped back beyond the arc, received the kick-out pass, and drained the go-ahead 3-pointer over Brunson’s outstretched arms. The Garden fell silent as the Celtics took a dramatic lead, sealing a victory that will linger in the Knicks' memory for weeks to come.