Duncan finished with 11 points, five blocks and two rebounds in 26 minutes and left for good with 4:15 left in the third quarter, when the Thunder led 67-48.
"They were excellent," Duncan said. "They were great. They started out strong. I thought we kind of weathered that storm early on, but they kept it on.
"We expected that kind of response from them in the third game and we didn't respond really well to that."
Parker finished with 16 points and four assists, but also committed five turnovers while being hounded by Thunder standout Thabo Sefolosha, who finished with 19 points, six rebounds, six steals and one blocked shot.
"They played harder than us tonight and deserved the win," Parker said. "They just played better. We have to play a lot better if we want to win here."
Stephen Jackson kept the Spurs within striking distance, scoring 16 points and shooting 6 for 7 from the field, including 4 for 5 from 3-point range.
"They flat out beat us," Jackson said. "They came out with more energy. They played better defense and knocked down shots. They just gave us an old-fashion butt-whipping."
Of the 20-game winning streak, Jackson said: "We're human. We had a good run. It's just one loss. We have just got to get ready to play the next game."
Game 4 is Saturday at 7:30 p.m. at The Peake. If the Spurs lose for the second time in their last 22 games, the series will be tied at 2.