MACAU Tennis legend Andre Agassi sees Andy Murray, Novak Djokovic and Juan Martin del Potro at the forefront of a changing of the guard in men's tennis.

But the former world number one believes Roger Federer and Rafael Nadal still have plenty to offer, and they could go down as the greatest players in history.

The American, who won eight Grand Slams before retiring in 2006, said the game was in a golden era and was only going to get better.

'It's a great time for tennis,' he said in a teleconference ahead of the Venetian Macau Showdown here next week with old rival Pete Sampras.

'Rafa is struggling with a knee injury but he can overcome it. Federer winning 15 Grand Slams and the French Open and his rivalry with Nadal is great for the game.

'If Nadal is able to win the US Open he will have won all four Grand Slams in addition to the Olympics and Davis Cup and one would be able to argue that the two best players in the history of the sport have come from the same generation.'

While Agassi had lavish praise for the world's top two players, he said the time would come when Murray, Djokovic and Del Potro take over.

'There is a possible changing of the guard with players like Murray, Del Potro and Djokovic and the game will only get better,' he said.

'They have the skill to overtake them (Federer and Nadal).' He pinpointed Britain's Murray as the pick of the bunch. 'Murray should win multiple Grand Slams,' he said.

'I predicted Murray to win at Wimbledon and the US Open. What you can't predict is the mindset.

'After Del Potro's win at the US Open in New York, it will light the fire of Murray and Djokovic and push them to get even better quickly.'

While Agassi likes what he sees from those players, he admits American men's tennis is in the doldrums.

'Tennis has become a bit lost in the US. We have 300 million people that can pick up a racquet. We just need to start getting more people playing,' he said.

'Other countries are pouring a lot of money into grass roots. But everything comes in stages and I am sure our time will come again — we've had many generations of success and Grand Slams.'