Novak Djokovic has lost the tenacity which enabled him to rule men's tennis for a number of years, former mentor Niki Pilic has said.
"Djokovic reached the Mount Everest of tennis last year after winning the French Open," Pilic, who guided the Serb as a junior at his Munich academy, told Bosnian daily Nezavisne on Saturday.
"He had 16,950 points and that's a tally which will never be repeated again, but loss of form and missing tournaments made his advantage melt away."
Djokovic looked untouchable after capturing his 12th grand slam title at Roland Garros last June, but was dethroned from the top of the world rankings in November by Andy Murray, who beat him in the final of the season-ending ATP World Tour Finals.
The 29-year-old appeared to have rediscovered some of his form when he gained revenge over the Briton in January's Qatar Open final but he then suffered a shock second-round Australian Open exit at the hands of Uzbek Denis Istomin.
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This week he was beaten by Australian Nick Kyrgios in the quarter-finals of an ATP event in Acapulco, prompting Pilic to cast doubts whether Djokovic can reclaim the top spot.
"Novak had a physical and mental edge second to none, he was in sixth gear," said the 77-year-old Croatian who helped a Djokovic-led Serbia to their 2010 Davis Cup title in an advisory role.
"That tenacity is no longer the same. It remains to be seen whether he can rediscover it and get back to the top level.
"The hard work of the last five or six years has taken its toll. Tennis was the priority every morning and afternoon and evening and all I can tell him is to be the person he was."
