World number one Novak Djokovic tested positive for COVID-19 on Tuesday, days after he hosted a number of players in the Adria Tour exhibition tournament in the Balkan region.Djokovic was criticized for hosting the tournament amid a pandemic after Grigor Dimitrov, Borna Coric and Viktor Troicki also returned positive tests after playing in the tournament that was held in Serbia and Croatia. Following are reactions from the tennis world to the Serb's positive test:Nick Kyrgios, two-times Grand Slam quarter-finalist:"Prayers up to all the players that have contracted COVID-19. Don't @ me for anything I've done that has been 'irresponsible' or classified as 'stupidity' - this takes the cake," the Australian wrote on Twitter. Andy Murray, former world number one:"Obviously it's not surprising how many players have tested positive when you see the scenes and the images and the videos from the tournament and the players party with no social distancing in place," Murray said at the 'Battle of Brits' tournament."I've seen some people say this puts the US Open in doubt but the measures and the protocols they have in place are completely different to what was going on in Serbia and Croatia."For a start, there will be no fans and the players will now know we can all be affected by this, it doesn't matter who you are, we need to respect the rules."Martina Navratilova, 59-times Grand Slam champion:"Yikes this is not good and it's a pattern. Hope Novak will be ok of course! What now, US Open? Roland Garros? We have a lot of work to do," she wrote on Twitter.Patrick McEnroe, former doubles champion and commentator:"When I sent out a tweet a couple weeks ago about that event in Serbia I said yeah it looks like a normal tennis event to me. I'm not sure everyone got my point," he said in a video on Twitter."Maybe they are starting to get it now. Nothing's foolproof so don't be foolish. That's my message."