Durant, who's known more for his three NBA scoring titles than for stepping in the way of oncoming opponents, got in Harden's way on a transition drive in the third quarter in a standout defensive effort for the Thunder against the league's second highest-scoring offense.
"It was all about giving your all every possession," Durant said.
Hours before going head-to-head with Harden, Durant said that "we're not looking at him as a friend right now." But even right before tipoff, the situation didn't seem natural.
"It felt a little awkward just looking across there during the national anthem and seeing him with another team, but we've got to get past that. We're competing against his team, and he wants to win so bad," Durant said. "We know him personally, so we know how hard he competes and he knows how hard we compete."
After letting Houston wipe away an 11-point deficit in the first half, the Thunder regained control with a 14-1 surge just before halftime and kept pouring it on.
Just after a fan nailed a half-court shot to win $20,000 in the break after the third quarter, Durant drove for a two-handed slam while getting fouled and Oklahoma City was soon up by 30.
"We didn't have a rhythm as a team. I felt like it was basically one on five every time," Harden said.
Harden, playing against the team that traded him away just before the season started, had 20 points but the Rockets were held 15 points below their regular-season scoring average while shooting 36 percent. Houston, which set a franchise record for 3-pointers during the regular season, finished 8 for 36 (22 percent) from behind the arc.
"We were trying to be solid on the defensive end," Durant said. "We knew they were a great scoring team. We just tried to pack the paint and get out to their shooters. We had to make the second and third efforts, and I think we did that."
Game 2 is Wednesday night in Oklahoma City.
"Believe it or not, I think this was good for us," Harden said. "Losing like this was definitely good for us. Now we know how to play."
"We really don't have an excuse anymore," said Jeremy Lin, one of three Houston starters making their first career playoff apperance. "We've gotten it out and played terrible all the way across the board -- offensively, defensively. That one's over with."
The Thunder were already up by 23 when Larry Hill connected on the half-court shot and went bounding toward Oklahoma City's huddle in celebration, getting congratulated by Thabo Sefolosha before the team's bison mascot pulled him back away.
Durant's dunk sparked a string of seven straight points for Oklahoma City, and Harden never even came off the bench in the fourth quarter. Coach Scott Brooks called timeout and pulled Durant a few moments later, and reserves finished out the game for the Thunder.
"We wanted to take care of our home court," Durant said. "We didn't do anything yet."
Russell Westbrook ended up with 19 points, 10 assists and eight rebounds. Serge Ibaka chipped in 17 points and Kevin Martin, Harden's replacement in the sixth man role, had nine of his 16 points in the fourth quarter with the game already in hand.
It was an impressive playoff opener for top-seeded Oklahoma City, which made it to the NBA Finals last season before losing to Miami in five games. The Thunder led by as much as 35 before Houston's reserves cut into the lead in garbage time.
Harden didn't get much help from the rest of his starting lineup, which features players with a total of three career playoff starts, all by center Omer Asik. None of the other starters scored in double figures and the unit went 17 for 50 from the field, with Harden making just six of his 19 attempts.
It was Houston's first playoff game in four years.
"James, on this team, has to shoulder a ton of responsibilities for us and he's been great all year long," coach Kevin McHale said. "He's had an all-pro type of year. ... He'll play better, just like everybody.
"We ask him to do an awful lot for us -- handle the ball, score the ball, pass the ball, make plays. I think that he had it going there for a little bit, but it wasn't James. It was us in general as a team. We never really caught a flow at all tonight."
Oklahoma City scored the first six points of the game and opened a 13-2 lead as the Rockets failed to make a basket for the first 6 minutes of the game and started out just 1 for 13 from the field. Houston eventually settled in and charged back with a 13-2 push spanning the break between the first two quarters, with Chandler Parsons providing the final five points on a two-handed putback jam and a 3-pointer.
Patrick Beverley pulled the Rockets even at 38 with a transition 3-pointer from the right wing, but the Rockets couldn't pull ahead before Oklahoma City's next surge.
Sefolosha had a two-handed slam on a fast break and also assisted on Kendrick Perkins' two-handed dunk during the burst, which extended Oklahoma City's lead to 60-45 before Harden's driving layup in the final second before halftime.
All but the first basket in the Thunder's run came after Asik joined Parsons on the bench with three fouls apiece.
The Rockets never recovered.
The game ball got switched out in the second quarter after it went into the stands and a drink was spilled on it. ... Harden scored a career-high 46 points in the Rockets' only regular-season win against Oklahoma City, but said he didn't gain any secrets from that performance to carry over into the playoffs. "I'm not going to lie. I was just making shots," he said. ... The Thunder improved to 18-5 in playoff games in Oklahoma City.
Information from The Associated Press was used in this report.