Federer, who pulled out of the US Open, said in August he would be sidelined for many months because he needed more knee surgery in order to resume playing
Djokovic is playing in his 10th semi-final at Wimbledon and 41st at the majors while his progress at the event has comfortably taken his career earnings past the $150 million mark.
Two-time champion Andy Murray, whose ranking has slipped to 118 as he battles to come back from hip surgery, will be aiming to win gold for the third Olympics in succession.
"I probably would have taken the place I am today," the 39-year-old, who pulled out from the French Open last week to protect his body after a gruelling third-round win.
"The knee just needs always extra care, and I hope it's also going to react well to the grass," he said, adding that he feels "all my strengths get amplified" when he plays on grass.
The American star's match will though be the first of this year's tournament held behind closed doors as France still has a 9pm curfew in place due to Covid-19.
