Roger Federer faces a demanding physical challenge if he is to win a ninth Wimbledon title, while Serena Williams' coach Patrick Mouratoglou makes Novak Djokovic clear favourite.
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 grass-court tournament he has won 10 times, succumbing rather meekly in the end to young Canadian Felix Auger-Aliassime.
His lack of match-fitness will be severely tested not only by Djokovic - who will be eager to flatten anyone who tries to stop him from joining Federer and Rafael Nadal on 20 majors - but also by an army of younger and fitter players.
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"I don't think he is satisfied yet with his level of play. He lost early on grass in Halle, which is something he's not used to and he was not happy," Mouratoglou, who has coached Williams since 2012, told Sky Sports.
"First he has to find his level completely because he's come back to tennis not so long ago after being injured for a long time so he's still looking for these details to make sure who he is on grass. And second is the physical aspect.
"To win a Grand Slam at his age in five sets it's such a physical challenge and facing guys like Novak in five sets, you know how physical that is.
"So that's the two biggest challenges for him more than the pressure."
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Djokovic is very much the player to beat and, having won the Australian Open and French Open to start the season, his chances of winning a calendar Grand Slam are growing stronger by the Slam.
Mouratoglou makes the world No 1 clear favourite to retain his Wimbledon title.
"Novak is definitely favourite to win Wimbledon, whether I make him or not," he said. "He knows he can win the calendar Slam or golden Slam.
"He also knows he can take over from Nadal and Federer in the Grand Slam stakes and that's his goal - to be the one with the most Grand Slams so that's going to be the biggest challenge for him.
"He has to deal with that pressure, which is the position he is at right now."
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