Boris Becker: Lack of characters worries Novak Djokovic coach

Boris Becker started working with Novak Djokovic before the 2014 Australian Open

Men's tennis could suffer from a lack of personalities after Roger Federer and Rafael Nadal retire, says six-time Grand Slam winner Boris Becker.

German Becker, 47, coaches world number one Novak Djokovic, who has been a mainstay of the men's top four with Andy Murray, Federer and Nadal.

Becker said the game is in a "healthy state" but added "it's impossible to carry on with only Novak and Andy".

"In two or three years' time, we have to be careful," he said.

"You have a young group of Australians - Nick Kyrgios and Thanasi Kokkinakis - who have good personalities and are interesting to watch, but there's a gap in between of players you don't know anything about."

Switzerland's world number two Federer, at the age of 33, is showing no signs of considering retirement and will target a record eighth Wimbledon title when the tournament begins on Monday, 29 June.

Spanish 14-time Grand Slam winner Nadal is only 29, but has been susceptible to injuries and is currently ranked 10th, his lowest position for more than a decade.

Becker, who became renowned for his charismatic approach after winning his first of three Wimbledon titles at the age of 17, believes the microscope of social media and intense news coverage has sterilised players' personalities.

"Nowadays everything is so supervised and so observed and everybody is very judgemental," he said.

"When I was playing you could be a little bit more emotional on the court and in your private life.

"I admire very much the current crop of players and I think the big four are exceptional players with exceptional personalities, but there are so many more in tennis who cannot really show their true sides because they are so protective and so careful."