BOSTON – In the spring of 2015, Al Horford believed he had a shot at his first NBA finals. Horford’s Atlanta Hawks finished the season with 60 wins, powered past Brooklyn and Washington, and entered the Eastern Conference finals with home-court advantage.

There, they faced the Cleveland Cavaliers. And the Hawks got swept. Horford’s stat line in the Game 4 clincher: Two points, two rebounds.

Six games into these conference finals, and familiarity has bred (some) contempt. Jayson Tatum has been manhandled by the Cavs’ perimeter players, leading to some frustrating moments. Jaylen Brown, a role player in Cleveland’s five-game beating of the Boston Celtics in last year’s playoffs, has battled with LeBron James, even needing to be separated from James after a battle for a rebound in Game 6 got a little too physical.

They are starting to dislike the Cavs. Horford? Horford has to hate them.

No Celtics player has as deep — or as ineffectual — a history with Cleveland as Horford. In 2016, Horford played in all 82 games. The Hawks won 48 of them, and marched into another matchup with the Cavs in the second round. Again, Atlanta was swept. In Game 4, Horford scored a respectable 15 points, but grabbed just four rebounds and missed four of his five 3-point attempts.

How about last season — Horford’s first in Boston. The Celtics charged into the conference finals with home-court advantage … and were eliminated by the Cavs in five games. Horford’s line in Game 5: Eight points, three rebounds.

These matchups with Cleveland haven’t been just bad for Horford — they have been nightmares. Dig deeper, and they get worse. Tristan Thompson, Horford’s longtime nemesis, has dominated his matchups with Horford. This series has been no different. Horford’s per-36 minutes with Thompson on the court: 9.1 points, 7.8 rebounds on 32 percent shooting. With Thompson off: 22.7 points, 9.7 rebounds on 62 percent shooting.

Game 6 represented a chance for Horford to exorcise his Cleveland demons, and he came up short. He went scoreless in the first half, and was part of the defense that allowed the Cavs to turn a five-point first-quarter deficit. This time, it wasn’t just Thompson, but multiple defenders Cleveland rushed at him.

“I was getting doubled as soon as I caught the ball,” Horford said. “So every time I didn’t really have one-on-one position. They did a really good job of doubling, making it hard.”

On the perimeter, too. Horford, a high-level outside shooter, is one of Boston’s most reliable options on pick-and-pops. In Game 6, the Cavs rushed him with different defenders to force him off his spots.

“They’re late switching — what we call veering — a lot of pick-and-rolls when [Horford] does set them with Thompson,” Celtics coach Brad Stevens said. “Thompson is tremendous defensively, but one of the things that makes him tremendous is his ability to switch. George Hill is into Al’s legs. And right on the catch or on the dribble, they are going, so they are doubling. If [Horford’s] touching the ball and making plays out of the double, then his number of field-goal attempts are going to be less.”

It’s been said before: Horford’s value to Boston goes beyond box scores. He’s a facilitator on offense, and the lynchpin of the Celtics’ top-ranked defense. Boston’s coaches cite Horford as a role model for Brown and Tatum, and his leadership has navigated the Celtics through one catastrophic injury after another.

“He’s been such a stabilizing force since he walked into our locker room,” Stevens said. “He provides a very calming influence to the younger players.”

Yet the Cavs are Horford’s Achilles’ heel, his kryptonite, the white whale he has chased for 11 years, with no success. The Celtics believe Thompson’s defense on Horford can be overrated, but there is little doubt that against Boston’s favored small lineup, Thompson’s physical play has an impact. And, as Thompson said earlier in the series, it’s a matchup he asked for.

Game 7 on Sunday means a lot of things to a lot of players. For LeBron James, it’s a chance to get to his eighth straight NBA finals — and extend what could be his last season in Cleveland a little longer. For Brown, Tatum and Terry Rozier, it’s a chance to shock the world.

For Horford, it’s different. At 31, Horford is coming off one of his most complete seasons, and the youth of the Celtics guarantees they will be back in this position next season. But he plays for a team that won’t hesitate to trade anyone, so for Horford, these opportunities can’t be wasted.

For years, the Cavaliers have stood in front of Al Horford. On Sunday, he may not get a better chance to climb over them.

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