Manu Ginobili had a chance to tie the contest with 4.9 seconds remaining, but his three-pointer from the left wing was off the mark. Inserted into the Spurs' starting lineup for the first time this postseason, Ginobili poured in a game-high 34 points.
"It wasn't a great shot, but it wasn't a bad one," Ginobili said of his tying three-point attempt. "It just didn't go in."
For the first time in this conference final, the home team failed to hold serve.
Just a week ago the Spurs looked unbeatable and hadn't lost in 50 days. But on the heels of their 20-game winning streak, the Spurs lost their third straight game for the first time all season.
"Championship teams win on the road," Spurs head coach Gregg Popovich said. "If we can't do that Wednesday, then we're not championship caliber."
Harden's four-point play with 5:17 left in the final frame capped an 8-3 Thunder spurt and gave the visitors a 101-88 advantage.
But the fiesty Spurs didn't go away and used an 11-0 run to climb back into the contest.
Stephen Jackson drained a three-pointer, Tim Duncan converted a pair of easy buckets before he and Ginobili hit a pair of free throws to cut the deficit to 101-99 with 1:51 left.
Westbrook swished a pull-up jumper from the right elbow, which preceded Duncan's turnaround bank shot.
Harden then hit the biggest shot of the game.
With the shot clock winding down and San Antonio rookie Kawhi Leonard in his face, Harden rose up and nailed a triple from the left wing, giving the Thunder a 106-101 edge with 28.8 ticks to play.
"Leonard was playing great defense on me. I just shot it with confidence. West Conference final, that's a big shot," Harden said.
Once again, though, the resilient Spurs didn't back down.
Ginobili drove in for a layup and Leonard made a fantastic defensive play to help San Antonio regain possession, knocking the ball off of Oklahoma City's Thabo Sefolosha along the right sideline with 15.8 seconds left.
With the Spurs down three, Ginobili, who went 5-of-10 from beyond the arc, missed the shot that counted most.
He received the ball from Duncan on a dribble handoff along the left wing. Fading a bit to his right, the Argentine let the ball go and backpedaled with his left hand raised in the air, but the shot clanked off rim.
Durant hit a pair of free throws with 0.8 seconds remaining to seal the win.
Shot-blocking specialist and one of the heroes in Game 4, Serge Ibaka, picked up two quick fouls in the first 44 seconds of the first quarter and the Spurs took an early 11-4 advantage.
But San Antonio, which went 6-of-7 from the foul line in the first 4:03 of the stanza, failed to capitalize and didn't attempt another free throw for the remainder of the period. Oklahoma City, meanwhile, ended the frame on an 18-6 run, taking a 26-21 edge into the second period.
The Oklahoma City margin swelled in the second quarter. Daequan Cook nailed back-to-back triples and poured in eight consecutive Thunder points at one point. A bit later, Harden's three-point play gave Oklahoma City a 50-36 lead before taking a 52-44 advantage into the break.
Ginobili sparked the Spurs during the first six minutes of the third period. He nailed three triples and scored 13-of-15 San Antonio points during a dominant stretch as the home team led, 67-61, midway through the frame.
But the Thunder closed the stanza on an 11-1 burst and took an 81-72 margin into the final 12 minutes. Durant had four points during the surge, including a jumper as the third-quarter horn sounded.
The Seattle SuperSonics fell to the Chicago Bulls in six games in the 1996 NBA final ... The Thunder were 8-of-21 from beyond the arc while San Antonio was 9- of-24 ... The Spurs outrebounded the Thunder 42-34 ... San Antonio was 26-of-31 from the charity stripe while Oklahoma City finished 20-of-23.