By Martyn Herman
LONDON (Reuters) - Tennis fans should intoxicate themselves on the elixir still being served up by the best men's generation to grace the game before the well dries up and mere mortals take over, at least that's what the doom-mongers think.
They say when 17-times grand slam champion Roger Federer, claycourt king Rafael Nadal and Serbian winning-machine Novak Djokovic, not to mention the mercurial Andy Murray, hang up their rackets quality, and interest, will suffer.
However, a hungry bunch of young lions are showing all the signs of keeping the flame burning long after the Fab Four settle down to a quiet life polishing their silverware.
At Indian Wells this year the men's Tour launched its ATP#NextGen campaign to promote rising stars aged 21 and under.
They were not short of names.
Volatile but dazzling 21-year-old Australian Nick Kyrgios, Borna Coric, a Croatian teenager with uncanny similarities to a young Djokovic and Alexander Zverev, a German already being compared to Boris Becker, are already established in the top 50.
Throw teenagers Hyeon Chung (South Korea), Karen Khachanov (Russia) and Quentin Halys (France) into the mix alongside American 18-year-old Taylor Fritz, the youngest player in the top 100, Japan's Yoshihito Nishioka, Britain's Kyle Edmund and Swede Elias Ymer and there appears to be critical mass of youngsters boasting 'future grand slam champion' potential.
Five years ago there were seven players aged 21 and under in the top 150, today that figure has risen to 13.
That said, there are no guarantees, according to American Brad Gilbert, a former top-10 player and coach to the likes of Andre Agassi and Murray.
"To become a great player and world number one and grand slam winner, you already have to be great when you're a teenager," Gilbert told Reuters.
"But I believe that there will be someone in this current group who steps up."
Stepping up to join Federer, Djokovic and Nadal, winners of 17, 14 and 11 grand slam titles since 2003, has been notoriously difficult, not just for rising stars, but everyone else.
Since 2010 Murray and Stanislas Wawrinka have won two apiece and Marin Cilic one while Juan Martin del Potro is the only other active player to have won a major.
"It's been tougher to make really big breakthroughs because the Big Four have sustained their excellence for an insane amount of time," Gilbert said.
Kyrgios, a Wimbledon quarter-finalist two years ago aged 19, is leading the charge having risen to a career-high 19th in the world heading to the French Open. He won his first ATP title this year and made semis in Dubai, Miami and Estoril.
Coric reached his first ATP final in India at the start of the year, while Zverev twice crushed former big hope Grigor Dimitrov and almost beat Nadal at Indian Wells.
Zverev cautions against predicting a decline for the big guns any time soon but says he is part of exciting times.
"There are a lot of stories about me being potential number one, about Kyrgios being a potential number one, about Coric, but what I think that shows is there is a very strong group of teenagers or young guys coming up," he said.
"I think we can all do great."
Federer is regularly asked his opinion of who will be the future stars and likes the look of Zverev, having beaten him in Rome last week.
"He will only get better from here," the Swiss said. "He will compete at the top, top 10 at least, if not then top five. Number one is always a big ask. A lot of things need to align for that: consistency, fitness, mental."
With world number one Djokovic turning 29 this week, a few days after Murray, Nadal 30 and Federer soon to be 35, ATP chief Chris Kermode is eagerly waiting to see how the tyros shape up.
"It's exciting," he said. "I've lived through the Borg-McEnroe era, Becker and Agassi and Sampras, people said things would never be the same again.
"But Kyrgios, Zverev, Chung, Coric, they are coming and the great thing is that they have such global appeal. They are at the cusp and we'll see who makes the jump first."
(Editing by Pritha Sarkar)