Westbrook nailed a 16-foot shot, but Spurs star Tim Duncan answered with a bank shot with 50 seconds left.
The Thunder ran down the shot clock and Harden launched a 3-pointer that swished with 28.8 seconds to go, giving OKC a 106-101 lead.
The Thunder had to avoid one last scare — an inbounds turnover with 15.5 seconds left gave the Spurs a chance to tie. But Manu Ginobili's desperation 3-pointer bounced off, and the Thunder had a monumental victory that could signal a shift in the NBA's balance of power, provided OKC can hold serve back home at Chesapeake Energy Arena.
“You know, championship teams win on the road, and Oklahoma City did just that,” Spurs coach Gregg Popovich said. “So they've proven they're a championship-caliber team. We have to go do that. If we can't win on Wednesday, we're not a championship-caliber team. It's as simple as that.”
The Thunder trailed 62-56 midway through the third quarter, but Durant, who only had one basket at halftime, got hot and keyed a 25-10 run.