Juan Carlos Ferrero, who is coaching Alexander Zverev, says there will be tighter competition between the veterans and newcomers at the top of the men's game.
"I think it is going to get really competitive among the Big Four and the young players in the Top 10 who want to settle down in the top of the rankings," he told the International Business Times.
And Zverev is among those young players, having won two Masters titles this season and reached No. 3 in the rankings.
"They now begin to see him as a potential number one. Because he is able to make it," said Ferrero, adding that he is now concentrating on developing Zverev's mental toughness. "We are working a lot on that side of him, so far Zverev has not had to face this aspect of the game."
But No. 1 Rafael Nadal and No. 2 Roger Federer are still formidable, said the former Grand Slam champ. Nadal won his 10th French Open this season and also the US Open, his first Grand Slam victories since 2014.
"He is one of the greatest in tennis history and his fighting capability is simply unbelievable," said Ferrero. “This year he has been playing more aggressively, his tennis is more straightforward, he is adapting his game style and he has been able to enjoy himself again on the court. A tennis player of his level is constantly trying to adapt and improve."
Federer also had among his best seasons, noted Ferrero, saying, "He has won almost everything he has played. He knows himself better than anyone, he knows where his limits are and he cherry-picks his calendar based on his knowledge on himself. One of the keys could be that Roger Federer has focused more on enjoying than on simply winning."
Turning his attention to the other two members of the Big Four, Novak Djokovic and Andy Murray, Ferrero warned of the difficulties of coming back from injury but also pointed to their quality of play.
"But it is possible. Again, they are among the greatest players on the circuit," he said. "And let's not forget players like Stan Wawrinka or Kei Nishikori, who have been injured for some months."
None of the four played the second half of the season because of injuries, but are now training for their returns to tour.