Novak Djokovic has lost the tenacity that 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 that 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.