PHOENIX — Russell Westbrook sprinted to steal the ball from Kevin Durant, sending Durant flying to the floor on his backside.

Durant winced in pain for a few moments before heading to the free-throw line at Phoenix’s Footprint Center as thousands of Suns fans in orange T-shirts held their breath. Westbrook quickly walked away from the scene, seemingly unconcerned, and waited for Durant to begin shooting.

The sequence played out like two ardent foes battling in an elimination game — which, technically, it was. Westbrook’s Clippers were on their last chance to stay alive in the first-round playoff series. But it was also a matchup between two men who had spent nearly a decade together as teammates, making the N.B.A. finals in 2012 with Oklahoma City as fledgling 23-year-olds tasked with carrying a new franchise in a small city.

“You know Russ is a fierce competitor, so when he sees K, it’s always about trying to play super hard,” said Suns guard Cameron Payne, who played with Westbrook and Durant on the Thunder.

Payne added: “Maybe in the regular season, he’ll go help him up, but you never know with Russ. Like playing with him in OKC, he was big on how it’s 15 guys on a team, and I’m with my 15 guys, so that’s just that competing stuff I was talking about.”

It was another puzzling moment in a jagged relationship.

Durant’s Suns won the decisive Game 5 on Tuesday, 136-130, holding off a late comeback attempt from the Clippers and advancing to the second round. Phoenix will play the top-seeded Denver Nuggets in the Western Conference semifinals beginning Saturday. Suns guard Devin Booker led all scorers with 47 points; Durant added 31, and Westbrook had 14.

Eleven years ago, Durant and Westbrook led Oklahoma City past playoff teams fronted by the future Hall of Famers Dirk Nowitzki, Kobe Bryant and Tim Duncan and headed into an N.B.A. finals matchup with LeBron James and the Miami Heat. The Heat beat the Thunder easily in five games, with Miami’s experience and star power proving too much for the upstart Thunder.

As swift as the loss was, it seemed to indicate that Durant and Westbrook would be back and would win championships together; they appeared too talented not to.

But they didn’t. Durant and Westbrook picked up individual accolades: Each has won a Most Valuable Player Award and many All-N.B.A. honors, but they never got to another finals together. Durant left for the Warriors in 2016 after the Thunder blew a three-games-to-one lead in the Western Conference finals against Golden State.

In their second matchup against each other after Durant’s departure, Westbrook yelled at his teammates and instructed them not to talk to Durant. They avoided questions about each other. Even former teammates like James Harden, who played with them in Oklahoma City, said they were “grown men” who had to “figure it out themselves.”

Since then, each has been on several teams. Durant won two championships with the Warriors and then headed to the Nets and now Phoenix. Westbrook has played on several teams that were supposed to have been title contenders — Rockets, Lakers, Clippers — but none has panned out. As Durant thrived, Westbrook began to be viewed as past his prime, no longer the player who could will teams to wins and average triple-doubles, and he has become the butt of jokes from fans when he struggles.

But in this playoff series against the Suns, Westbrook proved he could still be a difference maker. Westbrook signed with the Clippers in February as a free agent after the Lakers had traded him to Utah, where he was released to, at best, be the third option for the Clippers behind the stars Paul George and Kawhi Leonard. But injuries to George and Leonard made Westbrook the first scoring option against the Suns, and he often served as the team’s best rebounder and defender.

He finished the series averaging 23.6 points, 7.6 rebounds and 7.4 assists per game, looking like a version of his old self, and he had the game-sealing block on Booker in the Clippers’ Game 1 win.

“When he’s retired, people are going to really tell the truth about how they feel about his game,” Durant said after Game 4, when Westbrook had 37 points. “Right now, the fun thing to do is to make a joke out of Russ. But the way he’s been playing since he got with the Clippers is showing everybody who he really is.”

After Game 5, Westbrook reflected on Durant’s comments, with an introspective answer that sounded as if it could also serve as a pitch to teams to sign him this summer.

“I just think that I am a player that makes mistakes like anybody else,” Westbrook said. “I miss shots like anybody else. I turn the ball over like anybody else. But I also do a lot of things that a lot of people can’t do, and I’ve done a lot of things people haven’t done in this league.”

For Durant, this series, and these playoffs, have a different meaning in some basketball fans’ eyes: proving that he can win a title without Golden State’s Stephen Curry and as a team’s best player. Durant, however, has said that he doesn’t feel that pressure because he has “nothing to prove.”

The Boston Celtics embarrassed Durant and the Nets last season in the first round of the playoffs, sweeping them without much trouble. Boston’s star forward Jayson Tatum outplayed Durant, scoring a lot while also defending Durant.

And then, in the N.B.A. finals, Curry and the Warriors beat that Boston team that had easily conquered Durant’s earlier in the postseason.

In Tuesday’s win, Durant disappeared for much of the fourth quarter, going scoreless for nearly 10 minutes as Booker dominated the ball and the Clippers inched closer. As the postseason continues, how the Suns win — with Durant leading the way or with Booker, or someone else — will add fodder to the discussion about Durant’s place as one of the best players ever.

That was clear on Tuesday, as Suns Coach Monty Williams made sure to acknowledge in his postgame news conference. But Williams also said that he was at fault for Durant’s lack of touches at the end of the game.

“I’ve got to figure out ways to get him in space so he can catch the ball freely and be able to go,” Williams said.

As the game ended, Westbrook had many long embraces with Suns players and coaches on the court, but he never made it to Durant. Instead, Westbrook left the floor alone, with one hand raised to fans as he exited, while Durant did a television interview on the other end of the court.