MELBOURNE—Alexander Zverev believes there are probably more players at the Australian Open who have COVID-19 but they are not being tested, the third seed said on Wednesday. Frenchman Ugo Humbert said earlier on Wednesday that he had tested positive for the coronavirus, a day after his first-round defeat by compatriot Richard Gasquet, forcing him to isolate for a week in Australia. Players must be fully vaccinated to compete at the Australian Open, a rule that forced the government to deport world number one Novak Djokovic due to his unvaccinated status. “We are allowed to go outside to eat, allowed to do whatever we want, so I think it’s natural that more people get COVID,” Zverev told reporters after dispatching Australia’s John Millman in straight sets in the second round. “I think quite a few players had it when they arrived. Quite a few players I think have it now. We’re …