SYDNEY, Australia (Reuters) -- Lleyton Hewitt's bid to regain the No. 1 ranking from Roger Federer is doomed to fail, according to his former Australian Davis Cup teammate Pat Rafter.

Hewitt has hired Federer's former coach Tony Roche to try and help him get back to the top of world tennis but Rafter, who was also coached by Roche, said he had his doubts.

"It would be hard to imagine that Lleyton could get back to No. 1 in the world, but I still think he can have a crack at the top five," Rafter told the Australian Associated Press on Thursday. "I could be wrong, Lleyton might totally disagree and Rochey might disagree with me also, but the game has changed."

Hewitt won the U.S. Open in 2001 and Wimbledon the following season and was the youngest man in history to finish a year ranked No. 1 in the world.

He held the top ranking for a total of 80 weeks from 2001 to 2003 but has not won a Grand Slam title since and has slipped to 21st in the world.

Rafter said Hewitt's biggest obstacle to returning to the top was his comparative lack of power and size in a game being dominated by bigger players, although that did not mean he was a spent force.

Rafter believes Hewitt could still challenge for Grand Slam titles if he was prepared to make some changes to his game.

"I'm assuming Rochey's going to say to him he's got to change things up," Rafter said. "If he listens, the relationship's going to work, or if he doesn't listen the relationship's not going to work."

Hewitt has often said that it was Rafter who had inspired him to become a professional player after he was invited, as a young boy, to train with him in the lead-up to the 1997 first-round Davis Cup tie with France.

One of the few genuine serve-and-volleyers to succeed in the modern game, Rafter won the 1997 and 1998 U.S. Open titles and finished runner-up at Wimbledon in 2000 and 2001.

He played with and against Hewitt several times, including the 2001 Davis Cup final in Melbourne where Australia lost 3-2. Rafter pulled out of the deciding singles match with an injury and never played competitive singles again.

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