John McEnroe has supplanted Justin Gimelstob in the driver's seat on "the greatest road trip in sports" in an effort to settle that long-disputed tennis question: Who's the world's greatest tennis player? Roger Federer, McEnroe or Gimelstob?

Though he served as U.S. Davis Cup captain for one year, John McEnroe is not exactly the poster boy for the USTA. Yet the Hall of Famer and occasional USTA critic has come to the rescue replacing Justin Gimelstob in a series of Olympus U.S. Open Series ads in which the four-time U.S. Open champ challenges Federer for world supremacy and annoys Rafael Nadal from his seat on the U.S. Open bus.

The Wall Street Journal reports the USTA originally filmed the ads starring Gimelstob in March, but after Gimelstob's crass rant against women players in general and Anna Kournikova in particular, the USTA decided to remove Gimelstob from the ad campaign.

"It was heartbreaking because I put my heart and soul into those ads," Gimelstob told The Wall Street Journal. "The USTA gave me a great opportunity and I let them down."

Enter McEnroe who the USTA originally hoped to star in the ads, but feared it could not meet his asking price. USTA Chief Executive Arlen Kantarian, who is friendly with McEnroe, appealed to him to help out and McEnroe answered the call. He shot his scenes alone and he was edited into the spots in place of Gimelstob.  McEnroe told the Wall Street Journal he will donate his fee (a "miniscule amount" according to McEnroe) to his charitable foundation.

"They should have asked me in the first place," McEnroe. "The U.S. Open has always been close to my heart. I grew up in Queens."

In the scene with Federer, McEnroe holds a coffee cup emblazoned with the words: "World's Greatest Tennis Player." When Federer asks where he got the cup, McEnroe replies: "I've had this since 1981. Hey, you wanna play for it?"

"You keep it," answers Federer.

"Scared huh?" McEnroe retorts.

In the Nadal spot, the four-time French Open champion is behind the wheel of the U.S. Open Series bus, while McEnroe, clad in a matching Nike outfit, plays flamenco guitar and yells out "Ole!" as an annoyed Nadal shakes his head.

To view the commercials please visit this Wall Street Journal link.