NEW YORK -- A 13-point fourth-quarter lead had melted away and the Boston Celtics appeared to be on their way toward a Christmas to utterly forget.

With an opportunity to show that they belong in the conversation among the top three teams in the Eastern Conference to everyone sitting at home watching the first of the NBA's big holiday showcase slate of games, the Celtics built a 109-96 lead in Madison Square Garden midway through the fourth quarter.

But with a minute left, the Knicks tied the game at 112-112. And now Boston had to prove that it was resilient and clutch enough to be that next contender in the Eastern Conference after the Cleveland Cavaliers and Toronto Raptors.

So as the Celtics patiently worked the ball around the court and into Al Horford's hands in the post, Boston's biggest offseason acquisition was ready to take the game's biggest shot before he found Marcus Smart open in the corner. With the Knicks' defense rotating and collapsing on Horford, Smart calmly caught the ball and made a 3-pointer with the shot-clock winding down to push the Celtics up 115-112.

"That big 3-pointer kind of gave them the confidence to finish the game," Carmelo Anthony said of the Celtics.

Al Horford had 15 points, 7 rebounds, 5 assists, 3 steals and a dunk over 7-foot-3 Kristaps Porzingis. Nathaniel S. Butler/NBAE via Getty Images

The Knicks never recovered as Boston came up with two stops on the next possession, culminating with Avery Bradley's huge steal on Anthony with 18 seconds left to ultimately help Boston celebrate Christmas with a 119-114 victory.

These were the kind of fourth-quarter, game-deciding plays and stops the Celtics could not come up with enough against Horford's old team, the Atlanta Hawks, to get out of the first round of the playoffs last spring.

But the Celtics were eager to show the Christmas home audience that they're better now and the best of the crowded next tier of teams that trail Cleveland and Toronto in the East.

"We are not that comfortable [yet]," warned Isaiah Thomas, who scored 27 points, nine coming in the fourth. "We know we are getting there. We are going in the right direction. We got a long ways to go, but at the same time, with that starting five healthy, we know we can be scary in the East."

The Celtics have rebounded from a three-game slump earlier this month by winning five of their past six games. They now have 18 wins and are starting to rise in the standings out of a crowded gridlock of teams. The Hornets, Knicks, Hawks and Pacers are within three wins of Boston with the Bucks, Pacers, Bulls, Wizards, Pistons and Magic all within striking distance.

After Horford chose to sign with Boston in free agency, the Celtics were clearly supposed to be in the conversation with the Cavs and Raptors among the top three in the conference.

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"I think that we can be that good," said Gerald Green, who came off the bench and gave the Celtics a spark with eight points and four rebounds. "I think we haven't even tapped into all the way of how good we can be. Sometimes we tend to slide a little bit but I think we are a resilient group and we are just only getting better."

Injuries to Thomas, Horford and Jae Crowder have likely slowed Boston's ability to develop the type of chemistry general manager Danny Ainge has envisioned they will have. But a win like this could go a long way in helping the Celtics develop more trust and experience together.

On Sunday, Horford gave the Celtics a lot of what they were looking for and more when they gave him a four-year, $113-million deal. He had 15 points, 7 rebounds, 5 assists, 3 steals, 2 blocks and made a 3-pointer. And his impact may have been even greater than what his box score showed.

Offensively, the Knicks had to account for him. At one point, Horford pump-faked and blew by Joakim Noah at the 3-point line before driving and throwing down a surprising one-handed dunk despite Kristaps Porzingis coming over to challenge at the rim.

"They were closing out real hard and I was just going to go shoot it and that just created lanes for me," Horford said. "It was just one of those things that I was either trying to finish or I am trying to find my teammates. That's how you want to play. You want to keep the defense guessing."

No one would have guessed before this game that Horford would dunk on the 7-foot-3 Porzingis. But Horford kept the Knicks off-balanced, especially on the sequence that helped them fend off the home team at the end.

Smart moved the ball to Thomas, who didn't have a good shot with Anthony managing to stay with him. The ball went inside to Horford, who was ready to take a shot when he saw Smart. The Celtics signed Horford to stretch defenses, provide some inside scoring, rebounding and, as he showed in this case, passing.

Horford also is in Boston to add some toughness, evident by his minor run-in with Anthony when the two were handed technical fouls for jawing at one another in the third quarter. Neither would back down from the other.

"A Dominican and a Puerto Rican," Anthony laughingly cracked when asked about his technical with Horford. "Nothing serious. That happens when New York and Boston plays."

How Boston plays in the next couple of games will be interesting to see. The Celtics return home to face the gritty Grizzlies on Tuesday before a measuring-stick game in Cleveland on Thursday.

"Honestly, we have a ways to go as a group," Horford said. "We are up 10 there with a couple of minutes to go, blow the lead. There are some things we need to work on. But we have the chance, if we keep playing the right way, to keep growing as a group until the end of the year to be in that [top of the East] talk. Right now we're not there yet."

"But I think we're coming along," Horford added. "Definitely from the first game of the year to now, we've come a long ways as a group."