The Spurs were already down 24 points at halftime, and Tim Duncan was on the bench to start the third quarter. If this was the final game of his illustrious career, this wasn’t how it was supposed to end.

Gregg Popovich would eventually turn to Duncan, knowing he would need him if San Antonio was going to have any chance to get back into the game, and the Western Conference semifinal series against the Oklahoma City Thunder.

Old Man River Walk got old before our eyes in this series. But Duncan looked like old reliable down the stretch. He showed he had a little left in his tank. Then Duncan ran out of gas.

The signature moment was Serge Ibaka blocking the lift-less Duncan’s layup attempt with a chance to bring the Spurs within nine with 3:12 to go. This won’t be the play that defines Duncan, but it will be the one everyone points to when they say the greatest power forward of all time should retire

The Thunder closed out the Spurs — and maybe Duncan for good — with a 113-99 victory in Game 6 to win the series 4-2. Oklahoma City moves on to play the reigning NBA champion Warriors in the conference finals, in what could be the most entertaining series of this postseason.

And you shouldn’t be surprised if the Thunder ousts Golden State. Not after this series. Not after Kevin Durant and the dynamic Russell Westbrook played like superstars are supposed to in the playoffs, and big men Steven Adams and Enes Kanter dominated the paint and the Thunder bench made a big impact

But before looking ahead, let’s reflect on Duncan and the Spurs, in case this was his last NBA game.

“How do you know?” Popovich said. “Do you know something I don’t know?”

It was rough series for the 40-year-old Duncan. He totaled 17 points in the first five games, including the first scoreless playoff outing of his career. Seventeen used to be what he would get in a half or a really good quarter. Some suggested that Popovich should sit down Duncan. That wasn’t going to happen.

The five championships the Spurs have won since 1999 would not have been possible without Duncan. So with the game and the series on the line, Duncan was on the floor for the final 17:42. He scored 11 of his 19 points in the second half. But it just wasn’t enough.

advertisement | advertise on newsday

“Timmy was playing really well so he we played him as much as we possibly could because he earned the minutes,” Popovich said. “He did a really good job.”

When the game ended, Duncan hugged the Thunder players, and ran off waving to the crowd. It may be it after all.

Duncan has a player option for next season, so he could decide to give it one more try, especially if the Spurs can re-load again. There have been rumblings that the Spurs will make a run at Durant. Duncan may want in, or maybe his time is up.

The Spurs need to do something to change up things. They had a terrific regular season, the best in franchise history with 67 wins. They looked like they had the veterans, the ball movement and the defense to beat the Warriors and add another piece of championship hardware to their trophy case.

But against the hungrier, more active and more athletic Thunder, the Spurs finally looked their age. So many times in the last several seasons, the prognosticators predicted San Antonio would slow down. It happened in this series.

NBA videos

Kawhi Leonard and LaMarcus Aldridge are great cornerstone players. But the Spurs needed Duncan, Tony Parker and Manu Ginobili to have throwback performances last night. They didn’t.

Ginobili, another candidate for retirement, scored five points and made his only field goal with 9:46 left in the game. Parker had six points and three assists and didn’t play the last 19:18. Popovich used another graybeard down the stretch, 40-year-old point guard Andre Miller, instead of Parker.

You’ve heard it before, but let’s say it again with feeling: the Spurs need to get younger if they’re going to remain one of the elite teams in the Western Conference. The Warriors aren’t going anywhere. Durant could stick it out with Westbrook and the Thunder and keep Oklahoma City among the best.

If this was it for Duncan, he won’t have the storybook ending that he deserved like John Elway or Peyton Manning. But this part would be fitting: he would have done it with little fanfare, and without a farewell tour. That was Duncan.

Duncan always gave the Spurs what they needed, and put the team first. He never sought the spotlight and never wanted the credit for the Spurs’ success. In fact, if it wasn’t for his extreme professionalism and selfless attitude the Spurs wouldn’t have had the success that they’ve had.

advertisement | advertise on newsday

That’s why Popovich went back to Duncan and let him finish the game. No one in the NBA has been more reliable than Tim Duncan the last 19 years. That’s what defines Duncan.