If size is becoming increasingly important -- players are much taller and stronger -- that means tennis has to fight to keep its prospects from joining more physical sports to which they would also be suited.
"It'd have been tough to have steered Kobe Bryant or LeBron James into tennis because they were such good athletes ... and the scholarships -- Bryant went from high school directly to the pros," Bollettieri said.
"It's not that a smaller person can't make it, but it's more difficult today."
Even though he is only 20, Ryan Harrison is coming to a make-or-break time in his fledgling career, according to U.S. sports journalist Douglas Robson.
"He really needs to make a move and I think he realizes that," Robson told Open Court.
"He's had some tough draws at majors but he hasn't been past the second round of a grand slam yet, he hasn't won an ATP Tour title, and a lot of players his age would've already passed those thresholds."
High cost of developing champions
The United States Tennis Association has been criticized in recent years for failing to produce successors to the last golden generation of male players, but Bollettieri supports the efforts of McEnroe's younger brother Patrick, who took over as head of development in 2008.
"They certainly are doing a lot, but it's tough to convince people when their son looks like he's going to be a helluva football player or basketball player," Bollettieri said.
He said the USTA could highlight the top 20-30 young players in the country, but it would then cost $3-7 million a year to develop them.
To attend one of Bollettier's academy programs, players will pay between $50,000-69,000 depending on age and whether they board.
"Pat is working his ass off getting more people playing the game, but what happens is when they get to be 13, 14 and have a lot of potential -- that's where the cost factor comes in and they have to come up with millions of dollars," Bollettieri said.
"We've got more people playing now, more youngsters, but in order to get the champions ... the people are blinded by what they see every day."
Courier, who won four grand slam titles in the early 1990s and topped the world rankings at the age of 22, is also loathe to blame anyone for the lack of American men's success.
"America has no ownership of the top ranking. It's a free-for-all," the U.S. Davis Cup captain told Open Court.
"Tennis is a very individual sport and I think it'd be very naive for a country to take the credit for individual players. Does America take credit for Tiger Woods? I'd say Earl Woods deserves the credit -- he's the one who really drove that.
"So in the tennis landscape, you look at the Williams sisters. Their father was a big driver, my family was a big driver, so I don't think we can put any blame on anyone."