BOSTON -- Giannis Antetokounmpo had 18 points in the fourth quarter of Wednesday night's season opener between his Milwaukee Bucks and the Boston Celtics at TD Garden.

But for the NBA's two-time reigning MVP, all he was thinking about postgame the point he didn't score, as his missed free throw with 0.4 seconds remaining was the difference in Boston's thrilling 122-121 victory.

"I'm upset about it," Antetokounmpo said after finishing with 35 points and 13 rebounds in 36 minutes, in his first game since signing a five-year supermax contract with Milwuakee earlier this month. "But you can't change it. So it's done.

"Hopefully when I'm in the same position, I can make the next one. That's the mentality you've got to have. But obviously there's a little bit of a weight on your shoulders [in those situations] because, if you miss, that's it for your team.

"I'm a winner, and I want to do whatever it takes for my team to win. But, you know, you learn from every situation that basketball puts you in."

There aren't many situations quite like the one that the Bucks and Celtics saw themselves in at the end of this game, in which those final .4 seconds took what felt like several lifetimes to finally drain off the clock -- and, as a result, were accompanied by a roller coaster of emotions on both sides.

The weirdness began when, with Boston having the ball down by one with 8.9 seconds left, the ball eventually wound up in Jayson Tatum's hands on the left wing.

With him staring down Antetokounmpo, the league's reigning Defensive Player of the Year, Tatum settled for a difficult sidestep 3-pointer -- one that was well-defended by Antetokounmpo - and that, when it left his hand, looked pretty far offline.

"I'm not sure we could have defended Tatum much better than we did," Bucks coach Mike Budenholzer said.

But then, in an early Christmas miracle for the Celtics, the shot banked off the backboard and fell straight through the hoop, setting off a raucous celebration as Milwaukee called timeout.

"Nah, I ain't try to," Tatum said of throwing his shot in off the glass. "The angle that I had, and knowing how tall he is, once I let it go I knew it was going to hit the backboard.

"But I didn't necessarily try. I didn't try to do that."

Whether he did or not, the result was the same. Still, despite those celebrations, however, those .4 seconds remained on the clock -- enough time for Milwaukee to get one, final shot off at the basket. And, after initially having Anteokounmpo inbounding the ball - who later said he was hoping to throw a lob to center Brook Lopez -- Budenholzer called timeout and, this time, Jrue Holiday took the ball out.

On his way onto the court, Antetokounmpo said he had a simple message for Milwaukee's new star guard: throw the ball up, and give him a chance to win a jump ball.

"I told Jrue, 'Just throw the ball high. Just throw it high, and I'm going to try to go get it,'" Antetokounmpo said.

Holiday, who finished with 25 points, six rebounds and three assists in his Boston debut, did just that -- and Antetokounmpo was fouled by Celtics big man Tristan Thompson as a result.

That gave him two free throws that, if he made both, would tie the game. But after cleanly making the first, Antetokounmpo's second attempt came up well short. The ball then ricocheted back toward him, deflected off a player and rolled harmlessly away, allowing the clock to expire and the Celtics to escape with a victory.

"I love that he's there, game on the line, opening night, going to the free throw line," Budenholzer said. "He's been working on it. He knows how important it is. I think it's, again, of course we would love to win, you want to come out, you want to have success, but there's so many good things that happened, including Giannis going to the free throw line with the game on the line. "You can't duplicate that in practice and he's just going to get better and better as we go forward." Antetokounmpo shot just 13-for-20 from the free throw line in clutch situations last year for Milwaukee, according to ESPN's Stats and Information Group, and his career percentage in those situations -- 68.2 percent -- is below that of his career free throw percentage overall.

But, despite his struggles in those situations historically, Antetokounmpo said he's excited to be in those situations with the ball in his hands with the game on the line, and is already looking forward to the next time he'll be there again.

"It's fun, because you learn from it," Antetokounmpo said. "The more you're in situations like that, the more you can succeed.

"I want to be in those moments. I want to [have the ball] down the stretch. I want to shoot the last two free throws, I want to shoot the last shot, because if you think about it, if you do that a thousand times, some of the times it's going to go in. Some of the times, you're going to be the hero.

"And, some of the times, you're going to miss. But you learn from it."