The Boston Celtics looked well on their way to a conference semifinals matchup with the Philadelphia 76ers. Up 3-1 in their first-round series against the Atlanta Hawks, they led Game 5 at home by 13 points with six minutes left.

And then Trae Young happened. The Hawks’ floor general scored 14 of his 38 points in the final 3 1/2 minutes and drained the game-winning, 30-footer with 1.8 seconds left, propelling his team to a 119-117 victory and sending the series back to Atlanta for Game 6 on Thursday (8:30 ET, TNT).

The Hawks scored 23 points on their final 12 possessions, and they did it by giving the Celtics a taste of their own spacing …

1. Celtics space and attack

Robert Williams III is a defensive force and a lob threat who can pull opposing defenders toward the basket, opening things up for his teammates. But the Celtics are most potent offensively when they’re spacing the floor with five shooters. And while Boston ranked second in 3-point rate (3PA/FGA) in the regular season, that potency isn’t just about 3-point shooting.

With five guys on the perimeter, the paint is open for one of them to attack. And the Celtics did that early and often on Tuesday. Their third basket of the game was a simple Jaylen Brown drive past Clint Capela, with no defensive help in the paint …

Playing five out can open the paint for cuts as well as drives.

Though he’s not a shooting threat, Williams was also often spacing the floor, allowing for attacks even when he was in the game. And the Hawks had some pick-and-roll success with Clint Capela setting screens for Young early on Tuesday. But both teams were more efficient offensively in Game 5 with their non-shooting centers off the floor.

2. Hawks drive and kick

Williams was on the floor for almost all of the final seven minutes of Game 5, and the Celtics’ offense stalled after they built that 13-point lead. Capela sat the entire fourth quarter — save the final 0.5 seconds — when he was asked to guard the Celtics’ inbounder after Young’s go-ahead 3. And without him, the Hawks’ offense looked a lot like the one that they couldn’t defend for most of the night.

The run began with Onyeka Okongwu setting a ball-screen for Bogdan Bogdanovic, who snaked the screen and got to the middle of the floor. Blake Griffin (Okongwu’s defender) was there, but Williams also had both feet in the paint, leaving John Collins open in the right corner …

With the Celtics’ bigs looking to stay in the paint, Collins has taken 28 (54%) of his 52 shots from 3-point range in this series, up from 34% in the regular season. He’s only 8-for-28 (29%) from beyond the arc, but he was 4-for-9 on Tuesday and gave the Hawks some life with that fourth one.

Collins added a couple of 2-pointers after that 3, with the first coming on a roll to the rim where the Hawks’ spacing forced Williams to rotate from the other side of the floor. When he got there, Collins was already too close to the basket for him to prevent a layup.

3. Young cooking the bigs

While the Celtics were in “drop” coverage on those first-quarter Young/Capela pick-and-rolls, they were switching in the fourth quarter. That put their bigs on Young, and he feasted. First, he put Al Horford on his heels and drained a step-back 3 to pull the Hawks within three.

On the next possession, Bogdanovic set a ball-screen for Young to change his defender from Marcus Smart to Brown, who’s not as good at getting around screens as Smart. Then Okongwu came with a second screen that flattened Brown, and Williams failed to step up, allowing Young to step into his easiest 3-point attempt of the night …

The Celtics actually doubled Young after a Horford switch two possessions later, but Young went away from the switch and was too quick for Horford, who fouled him. After the Celtics took back the lead, Smart unwisely fouled Young 50 feet from the basket. And the game-winner was another step-back against Brown before Smart could get there with another double-team …

Boston outscored its opponents by 13.2 points per game, the league’s biggest differential, from 3-point range in the regular season. And the Celtics were a plus-7.5 points per game from beyond the arc through the first four games of this series. But the score from 3-point range in Game 5 was Hawks 57, Celtics 36.

The Celtics ranked second defensively in the regular season, and they don’t have any real defensive weak spots in the top seven of their rotation. But Young is a special offensive player who can beat any kind of defensive coverage. The Hawks have the ability to spread that Celtics’ defense out, just like the Celtics can do on the other end of the floor. And often, it comes down to which team can do a better job of just staying in front of the ball and forcing more difficult shots.

If the Hawks can defend better in space than the Celtics on Thursday, they can force a Game 7.

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John Schuhmann is a senior stats analyst for NBA.com. You can e-mail him here, find his archive here and follow him on Twitter.

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