RETIRED Australian Davis Cup captain John Newcombe fears Mark Philippoussis could fail to capitalise on his awesome talent because of a lack of commitment.
And the former world No.1 believes Pat Rafter needs to improve his fitness to win a third major.
Newcombe warned US Open finalist Philippoussis will surrender the opportunity to join Rafter as a grand slam singles champion unless he dedicates himself completely to the sport.
"It all depends on Mark," Newcombe said.
"There's a couple of grand slams there for him but there's not a couple of grand slams there if he's not willing to commit himself to the task at hand.
"So far in his career he hasn't committed himself.
"He keeps compromising and getting away with doing about 60 per cent of the workload he should be doing and in that he does enough to stay in the top 15 in the world, but it's just a joke he's not in the top six.
"There's no doubt in my mind he's top six in the world if he didn't compromise."
Newcombe said he was baffled by Philippoussis's attitude to training, predicting the Victorian might never win a slam because of a lack of fitness.
"I don't know what it is, he has just not made the commitment that he's going to be the best possible player he can be," said Newcombe, winner of three Wimbledon, two US and two Australian singles crowns.
"Until he does that he's not going to get there.
"He won't win a grand slam because you've got to win seven best-of-five set matches and he can't do it if he's not fit enough."
Newcombe believes Rafter can win at least one more grand slam title provided the Queenslander improves his fitness.
"Pat's got to get fitter than what he is right now," Newcombe said.
"Maybe he has to change a few things because his style of game, which is such a physical style of game, means he has to have legs that go longer than anybody else.
"Right now he's confronting problems with cramping, so something has to be done."
Rafter retired from his singles match on the opening day of the Davis Cup final against Juan Carlos Ferrero because of severe cramping.
The dual US Open champion believes the cramping might have been related to a build-up of stress, created by watching Lleyton Hewitt battling Albert Costa for four hours and nine minutes.
Newcombe predicts world No.6 Hewitt, 19, can claim as many as "four or five grand slams, maybe more" provided he overcomes the breathing problem which threatens his progress.