"When you've got guys like Rafa, Novak and Roger playing at the level they're playing, there's not going to be a whole lot of winners outside of those guys and now you can put Andy Murray inside of that group as well.

"It's been very, very difficult for American tennis to be able to follow our generation."

Success breeds success

Bollettieri believes the success of his Florida-based academy was based on offering scholarships to the likes of Agassi, Courier and Monica Seles, which later attracted other young talent from around the world including current stars such as Maria Sharapova and the Williams sisters.

"When you get good people like that at the same place, when you get 15, 20, 30 players who are the best in America and some of the best in the world, they compete," the 81-year-old said.

"They're in a competition, not costing them a dime -- it helped get me where I am. You've got to have it very attractive for a young boy or girl of 11, 12, 13 years old to play tennis opposed to other sports if it looks like they're a hell of an athlete."

Roddick retired during last year's U.S. Open, on his 30th birthday, and now John Isner is America's No. 1 player.

A towering 6 foot 9 inches, he could have been a basketball player but stuck with tennis and kept playing through four years at the University of Georgia before turning professional. It's an unusual path, as most top players join the pro ranks in their teens.

Ranked 15th in the world after reaching a career-high ninth last March, he has never gone past the quarterfinals of a grand slam. Arguably his major claim to fame is playing in the longest match of all time at Wimbledon in 2010, though he has beaten Federer and Djokovic.

"I would love to go further than that (quarterfinals) and I know my fellow American players would love to as well," Isner told Open Court at this month's San Jose Open, which is folding after 125 years of existence at various Californian venues.

Opportunities for young American players are reducing, with the second-oldest tennis tournament in the U.S. relocating to Brazil, while the license for the Los Angeles Open -- which began in 1927 -- was sold to a Colombian group last year.

"Ten years, it is a long time, but I don't think American tennis is as bad as people portray it to be," Isner said.

"In the '70s, '80s, '90s, I think American tennis fans were a bit spoiled with all these great players. The game today is very, very tough. It's very international.

"You see players in Europe that are just so strong and so physical and these guys are really dominating the game, especially the players that come from Spain."

Size matters

Bollettieri believes Isner will struggle to win a grand slam, not because a lack of ability but because of his size -- though Juan Martin del Potro, three inches shorter, won the 2009 U.S. Open.

"John has a lot of the tools, but the bigger you are you have to have footwork as well. The downfall of John today is footwork, mobility," Bollettieri said.

He is wary of predicting a future male U.S. champion, but is encouraged by some of the young talent emerging.

Bollettieri points to Christian Harrison, the 18-year-old younger brother of world No. 76 Ryan -- who at just 20 has already represented his country in Davis Cup and passed $1 million in on-court earnings.

"Today the game depends on strength, mobility and then talent. You've got to have strength and speed, you've to have at least two weapons, you'll never make it with just one," said Bollettieri, who also has high hopes for a 14-year-old he calls a "big boy," though he kept his name secret.