After the most successful season in franchise history, the Spurs find themselves on the brink of elimination from the NBA playoffs after another crunch time collapse cost them a 95-91 defeat in Game 5 of their Western Conference Semifinals series against the Thunder at AT&T Center.

They must win Thursday’s Game 6 at Chesapeake Energy Arena in Oklahoma City or face a long summer wondering what happened to a 67-15 team that for months had seemed destined for a Western Conference Finals matchup against the defending champion Golden State Warriors.

Potential elimination on Thursday also carries with it the possibility Spurs fans have seen the last of Tim Duncan. The 40-year-old team captain, as certain a first-ballot Hall of Famer as any player in NBA history, endured another rough night at the offensive end in Tuesday’s loss. In 28 minutes he scored only five points and grabbed only three rebounds, missing 5-of-6 shots.

He logged more playing time than any of the previous four games of the series and was solid at the defensive end. But there is no hiding the fact he now has two aching knees that may conspire with one of the most frustrating playoff series of his splendid career to impel a decision to retire.

Duncan’s longest-tenured teammate, point guard Tony Parker, praised the big man’s energy and effort in Game 5.

“I thought he played great, tried to give it everything he’s got,” Parker said. “He had a couple big boards for us. He’s doing his best.

“We don’t think about (his future) right now. Right now it’s Game 6 and we have to play well. That’s it. Let’s keep him playing a little longer.”

Another Duncan game at AT&T Center could have been guaranteed had the Spurs just been able to protect a lead in the final four minutes on Tuesday.

Ahead by six, 88-82, after a Kawhi Leonard dunk, the Spurs were undone by the dynamic play of Oklahoma City point guard Russell Westbrook and their own horrid shooting. The NBA’s leader in triple-doubles this season, Westbrook scored 10 of his game-high 32 points in the final three-and-a-half minutes, when the Thunder outscored the Spurs, 13-3.

It was similar to the final three minutes of Game 4, when Kevin Durant sparked an 11-1 Thunder run that sealed a 111-97 win.

Westbrook also had 11 rebounds in Game 5, one of three Thunder players with double-figure rebound totals. Spurs coach Gregg Popovich lamented Oklahoma City’s advantage in offensive rebounds when the game was on the line.

“We lost a couple of boards there at the end in the fourth quarter just like last game,” he said.

The NBA leader in triple-doubles this season, with 18, Westbrook finished just one assist shy of another one. His ninth assist came with two minutes left, when his pass to reserve big man Enes Kanter produced a layup that tied the score at 88.

The Spurs missed eight of their final nine shots, including four wide-open mid-range jumpers and one layup.

“We just missed a lot of easy shots,” said LaMarcus Aldridge, who missed 15-of-21 shots, two of them in the final 2:40. “I thought guys had great looks. I had a couple shots I normally make that came out. I thought guys competed but the ball didn’t fall for us.”

The game was not without its share of refereeing controversy, though nothing as egregious as the five violations the NBA acknowledged the officials missed in the final 13.5 seconds of Oklahoma City’s 98-97 victory in Game 2.

This time, they appeared to miss Kawhi Leonard’s intentional foul on Westbrook after he took an in-bounds pass with 9.3 seconds left, the Thunder leading by a single point, 92-91.

Leonard reached out and contacted Westbrook with his left arm, but no foul was called and Westbrook continued to the basket and scored, converting a three-point play when Aldridge was called for a foul as he tried to stop the shot.

“I definitely fouled him,” Leonard said. “The referee, I guess he didn’t see it, and you’ve just got to keep playing.”

Popovich, who rarely comments on the officiating, felt compelled to mention the non-call of a foul he had ordered taken and a second officiating decision, a foul call on Danny Green when it appeared he had fallen into Kevin Durant after being intentionally tripped by Thunder center Steven Adams

“He fouled him,” Popovich said. “It’s pretty obvious he fouled him but every foul doesn’t get called. That’s the way the game is. I was more concerned about the play before that with Durant’s shot.”

Durant’s two free throw makes, with 54.7 seconds left, gave the Thunder a 92-90 lead and set up the drama that followed.

Green, whose 6-for-9 3-point shooting was wasted, was incredulous when asked if Adams’ trip was intentional.

“Is that a trick question?” he said. “I don’t know if it was intentional, I didn’t see the replay clear enough.

“On that particular play, he did a good job of clearing out. I tried to chase KD around the screen, but my leg came out from under me, I’m pretty sure it was his foot.  I don’t know if it was intentional or not, but the reason why I fouled Durant was because of the domino effect of being tripped.”

The final two-minute report the NBA will issue on Wednesday will be must reading for Spurs fans who thrive on outrage but it won’t solve anything for the team.

“Sometimes you get a call; sometimes you don’t,” Popovich said. “It happens to everybody. Tough game.”

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Mike Monroe, Longtime NBA and Spurs Writer, Still in the Game

Mike Monroe is a longtime, award-winning sports journalist who has covered the NBA for the San Antonio Express-News and other publications.