By Steve Ginsburg

MIAMI (Reuters) - Dirk Nowitzki scored a game-high 27 points and grabbed eight rebounds but the Dallas Mavericks power forward was the first to admit he was far from his best in the NBA Finals opener on Tuesday.

The Miami Heat beat the Mavericks 92-84 in a game that saw two of the NBA's most potent offenses struggle, and while Nowitzki connected on seven-of-18 shots and all 12 of his free throws, the German appeared more subdued than usual.

"I had my opportunities there," he told reporters. "I had some good drives there in the first half. I've just got to finish. I missed a righty (right-handed) lay-up. I missed a lefty lay-up that rolled around.

"I have to keep attacking and take my opportunities when they're there."

The Mavericks shot only 37.3 percent as a team but the Heat were not much better, managing a marginally higher 38.8 percent.

Miami's Udonis Haslem has the assignment of guarding Nowitzki and said his job is to "try to make it tough" for the 32-year-old German.

"He's a great player," Haslem said. "He's going to make his shots. You can't get discouraged. You've got to keep going and keep chipping away at it."

Miami coach Erik Spoelstra said he was unsure how to stop Nowitzki, who entered the game with a 28.4 average in the post-season.

"I don't think it really matters what you do, your schematics, who is defending him, he's going to get his average at least every single game," he said.

"It's uncanny, because of his shooting ability, his skill level. So I don't really know if we affected him that much."

Several Heat players harassed Nowitzki each time he touched the ball, a familiar sight for the 10-time All-Star.

"Every team we've played is very aggressive on him," Dallas coach Rick Carlisle added. "He's a unique player. He's got to be denied the ball virtually everywhere on the floor.

"Haslem is a guy that has the wherewithal to do that. And they're going to play him aggressively. And we know that.

"We just got to keep playing our game. We have to keep getting him the ball and giving him opportunities to create."

Game Two of the best-of-seven series is in Miami on Thursday.

(Editing by John O'Brien)