By Martyn Herman
LONDON (Reuters) - Only 26 men have scaled the top of the ATP world tennis rankings but for some of them it has been such a fleeting experience that their names have been reduced to obscure pub quiz questions.
How many outside the tennis fraternity remember Chilean Marcelo Rios, Austria's Thomas Muster or even Australian Pat Rafter, the former U.S. Open champion who achieved the feat for one week in 1999?
Triple grand slam champion Andy Murray is the latest to add his name to the list and former coach Brad Gilbert is backing the Scot to keep on improving.
"The challenge is within himself," said former world number four Gilbert who helped Murray reach the top 10 during a 16-month partnership that ended in 2007.
"Murray is so hungry," the American told Reuters in a telephone interview. "He wants to get four (slams) then he will want five.
"Lots of guys have touched number one but not finished the year number one. I guarantee he will want to stay there...and if he ends the year number one I guarantee he will want to do it twice."
But for a date of birth that thrust Murray into arguably the greatest ever era in men's tennis, the 29-year-old would surely not have had to wait so long to reach the summit of the sport.
When the new rankings are released on Monday, Murray will be looking down on Novak Djokovic, Roger Federer and Rafael Nadal, a three-man roadblock that has been in his way for the best part of a decade.
The trio have spent a combined 666 weeks as world number one but with age catching up on Federer and Nadal, Gilbert said Murray and Djokovic had a two-year window of dominance ahead.
GANG OF FOUR
"It used to be a gang of four now it's two. I can't see anybody in 2017 ascending the challenge other than Murray and Djokovic," he explained.
"Maybe in 2018 or 2019 Alexander Zverev or Nick Kyrgios one of those guys but I would say Murray and Djokovic have got a window for a couple of years. I don't see anything changing next year unless one of the younger guys makes a massive upgrade."
Gilbert said the catalyst for Murray to begin closing a yawning 8,000-point gap on Djokovic in the rankings was returning to work with former coach Ivan Lendl.
Murray re-hired Lendl after losing to Djokovic in the French Open final in June and went on to claim his second Wimbledon crown before landing Olympic gold in Rio de Janeiro and launching a late-season charge to catch the Serb.
"It's been an amazing run from Andy," Gilbert said. "I think the claycourt season laid the groundwork for where he is today. I've never seen him play better, more aggressive tennis.
"I think it was the time he added Lendl that was key. The timing of it was perfect. Everything was so positive."
Gilbert said the Scot would need the same level of consistency if he wanted to fend off an inevitable Djokovic counter-attack, beginning at the ATP World Tour Finals in London later this month.
"Murray is playing better at the moment but he has never made the final in London where Djokovic has a superlative record," he added.
"It's just about winning matches now for Andy, keep winning and things will take care of themselves."
(Editing by Tony Jimenez)