Of all the players on the Dallas Mavericks' roster, Shawn Marion has been given the most difficult assignment in this year's playoffs.

It's not as tough as riding a raging bull. But by NBA standards, what Marion has been asked to do over the past six weeks has been about as difficult as trying to contain Michael Jordan night after night after night.

Marion has gone from guarding Portland's Brandon Roy in the first round of the postseason to defending All-Stars Kobe Bryant (Los Angeles Lakers), Kevin Durant (Oklahoma City) and now Miami's LeBron James. That's about as tough as it gets in the NBA.

The rugged defensive challenges, however, don't faze Marion.

"Somebody's got to guard these guys, and I'm going to take the challenge to do what I've got to do and make it hard as I can for these guys," Marion said after Saturday's practice. "All of them are different and all of them get a lot of shots, so it's just a matter of how many shots are you going to let them get.

"I'm going to contest every shot I can and keep them off the free-throw line. It is what it is."

The Mavs are hoping Marion will be at his defensive best when they host the Miami Heat in Game 3 of the NBA Finals at 7 tonight at American Airlines Center.

This best-of-seven series is tied 1-1, and since the NBA went to the 2-3-2 format in 1985, the Game 3 winner is 11-0 when the series was tied after two games.

It's essential that Marion continues playing superb defense in order for the Mavs to win their first NBA title. His importance at the defensive end is unparalleled.

"We wouldn't be here without Shawn Marion's defense," Mavs backup center Brendan Haywood said. "His defense has been great, and that's probably one of the underrated parts of his game.

"He's really accepted the challenge to guard a lot of really, really tough offensive players in this league."

Other than defense, the Mavs also expect Marion to hold his own at the offensive end of the floor.