LONDON, June 24 (Reuters) - It is almost two years since Novak Djokovic left Roger Federer crestfallen by edging a momentous Wimbledon final but as the tournament prepares to return after last year"s cancellation, the men"s game remains in the Serb"s stranglehold. Since that epic five-set victory, in which he saved two match points and won the first fifth-set tiebreak in a Wimbledon final, Djokovic has claimed three of the next six majors with Rafa Nadal bagging two and Dominic Thiem one. It would likely have been four for Djokovic had he not been defaulted from last year"s U.S. Open -- allowing Thiem to sneak in and finally add a new name to the Grand Slam winners" club which has been ruled by Roger Federer, Djokovic and Nadal for over 15 years. That proved a mere bump in the road for Djokovic, however, and he will arrive at Wimbledon bidding for a sixth title to match Federer and Nadal"s men"s Grand Slam record haul of 20. Should he triumph he would also be three quarters of the way to completing the first men"s calendar-year Slam since Rod Laver scooped up all four in 1969. Nadal will not be playing at Wimbledon, having decided to rest, while eight-time champion Federer is showing signs that, at 39, the window of opportunity to win one more Slam is closing. Federer, who underwent two knee operations last year and has played only four tournaments since the 2020 Australian Open, was not his usual self last week at the Halle grasscourt tournament he has won 10 times, succumbing rather meekly in the end to young Canadian Felix Auger-Aliassime. Writing off his chances would be foolhardy, but the main threats to Djokovic might come from a young brigade gradually edging closer to shaking up the status quo in men"s tennis. "Is his level high enough to compete? Not in the end of matches but from the first point? That"s the question mark that I think only he knows the answer to until we see him play," seven-time Grand Slam champion mats Wilander said of Federer"s competitiveness ahead of Wimbledon. Djokovic looked supreme at times in Paris as he claimed his second French Open title -- but there were also dips. He lost the opening two sets against Italian youngster Lorenzo Musetti in the fourth round, dismantled Nadal in a breathtaking semi-final, but again found himself two sets in arrears in the final against Stefanos Tsitsipas. Therein lies the problem for those trying to stop the Djokovic juggernaut. Taking three sets off a player who is teak tough mentally and physically is one of the toughest challenges in world sport. "It"s all about endurance, if you can keep up with the endurance, keep your level there for longer periods of time, then of course that"s what is needed in a Grand Slam," Greek Tsitsipas, who will be a threat at Wimbledon if he can shrug off any French Open hangover, said. World number two Daniil Medvedev will take huge confidence from his run at the French Open, seeing that clay is his least favoured surface. His huge serve and flat groundstrokes will give him a shot, while Germany"s Alexander Zverev is another with the weapons to challenge for Djokovic"s crown. Big-hitting Italian Matteo Berrettini might be the dark horse, however, after winning the Queen"s Club title on his debut last week -- the first player to do that since Boris Becker in 1985. Becker won Wimbledon a few weeks later. And as crowds prepare to return to the All England Club, home favourite Andy Murray will get the biggest cheers. The two-time champion is struggling to return to anywhere near his best after hip resurfacing surgery in 2019 -- but few know their way around a grasscourt better than the wily Scot.