CLEVELAND — Boston playoff star Jayson Tatum and Cleveland coach Tyronn Lue are cousins. The Cavaliers and Celtics might as well be.

The Eastern Conference’s top two teams have been intertwined for more than a year — from a summer blockbuster trade, from opening night, from afar.

Although there were long stretches this season when it seemed at least unlikely, and maybe impossible, that the Celtics and Cavs would reach the conference finals, they’re back again and meeting in the playoffs for the third time in four years.

The series everyone wanted. The one few expected to see.

And yet here they are: the odds-defying Celtics, guided by baby-faced coach Brad Stevens and prospering without injured stars Kyrie Irving and Gordon Hayward, versus the Cavs, who despite a massive roster makeover in February are rolling as 33-year-old LeBron James enhances his legend with every game.

"There’s been a lot that’s happened throughout the season with us and for them," Cavs forward Kevin Love said Thursday as Cleveland prepared for Sunday’s series opener in Boston. "Both teams have raised their level of play and been through it, but are here now and have to lay it all on the line."

It’s hard to process how much has changed for these teams since they played last May.

Major moves, injuries, upheaval.

After the Cavs beat the Celtics in five games in last year’s conference finals to advance to their third straight NBA Finals, there were no hints the two franchises would ramp up their rivalry with a trade that caught their entire league, well, off-guard.

The Cavs succumbed to Irving’s trade demand — he was tired of playing in James’ shadow — and sent the All-Star to Boston for Isaiah Thomas, a first-round draft pick, forward Jae Crowder and centre Ante Zizic. The 5-foot-9 Thomas had bravely carried the Celtics in the 2017 post-season despite a balky hip and broken heart following his sister’s death in a car crash.

With so many subplots, the league scheduled Boston at Cleveland in the season opener.

However, Irving’s awkward reunion with James was quickly overshadowed by Hayward, Boston’s big free-agent signing, grotesquely breaking his ankle just feet from the horrified Cavs bench. As Hayward left on a stretcher, the Celtics’ season seemed to be riding to the hospital with him.

Boston, though, showed grit and reeled off 16 straight wins before injuries resurfaced. Marcus Smart had thumb surgery and the Celtics lost Irving, who needed more knee surgery and won’t get a crack at dethroning James.

Irving will watch this series from the bench. Thomas is much farther away.

He spent the first two months in Cleveland trying to get healthy, but wasn’t the same when he came back and never connected with his new teammates.

With Thomas a mess and the season slipping away, Cavs general manager Koby Altman traded him on Feb. 8 to the Lakers for Jordan Clarkson and Larry Nance Jr. Altman didn’t stop there, dealing Crowder and Derrick Rose to Utah for Rodney Hood, sending Dwyane Wade back to Miami and acquiring point guard George Hill from Sacramento.

Three days after the trade, the new-look Cavs beat the Celtics 121-99 in Boston as Hill, Clarkson and Hood combined for 44 points to the delight of James, who seemed rejuvenated after weeks of moping.

He’s now on a mission as he prepares to face the Celtics for the seventh time in the post-season. James is averaging 34.3 points, 9.4 rebounds and 9.0 assists in 11 games in these playoffs, which has included him making two buzzer-beaters, scoring 45 points in Game 7 to oust Indiana and dispatching Toronto with shocking ease.

"LeBron’s on just a ridiculous run of play," Stevens said. "We know that it’ll be quite a challenge."

The Celtics may not have anyone of James’ stature, but they have a complete team that’s clicking. Tatum and Jaylen Brown are two of the game’s rising young stars and Cleveland native Terry Rozier has emerged as a post-season menace, filling in for Irving and quickly winning over Boston fans, who have dubbed him "Scary Terry."

Lue knows the Cavs have their hands full.

"They’re well-coached," Lue said. "Despite all the circumstances of guys getting hurt with Kyrie out, with Gordon being out for the whole year, they just keep plugging guys in and keep playing. (Boston executive) Danny Ainge has done a good job of getting guys in that fit that mould of Celtics basketball and they’ve been playing well."

Especially the 20-year-old Tatum. He averaged 23.6 points in Boston’s series win over the Philadelphia 76ers, who didn’t do enough to slow him down.

That now falls on Lue, who has known Tatum — or "Little Jay" as he called him — since he was a kid. They’re related on Lue’s grandfather’s side of the family.

"He’s not scared, he’s not afraid and he’s always been that type of kid," Lue said. "He wants these type of moments and he’s taking advantage of it. With Kyrie and Gordon going down and him having a chance to play the minutes he’s been playing, starting and playing in the playoffs the way he has, he’s taken full advantage of it. But he’s never been scared, he’s never been afraid of the big moment. You can see that."

Six years ago, a 14-year-old Tatum posted a picture of himself, wearing a replica Kobe Bryant jersey, with James on Twitter and asked Cleveland’s superstar to follow him.

Lue smiled when shown a picture of his cousin.

"Now look," he said. "That’s crazy."

So is another Boston-Cleveland final.