BRISBANE, Australia—Reaction from social media and elsewhere to the Australian government denying No. 1-ranked Novak Djokovic entry and the cancellation of his visa because he failed to meet the requirements for an exemption to COVID-19 vaccination rules: “The world has been suffering enough to not follow the rules.”—Rafael Nadal, who is tied with Djokovic and Roger Federer with 20 Grand Slam singles titles. “If he had a fair exemption from the rule, well, he should be here; if he didn’t, he shouldn’t be here.”—Daniil Medvedev, who is ranked No. 2 behind Djokovic on the men’s tour. “I told our Novak that the whole of Serbia is with him and that our bodies are doing everything to see that the harassment of the world’s best tennis player is brought to an end immediately.”—statement from Serbian President Aleksandar Vucic. “He played by the rules, he got his exemption, he’s a nine-time champion, …