The action on court may be heating up at the Australian Open, but talk surrounding the first Grand Slam of the tennis season continues to be dominated by allegations of widespread match-fixing within in the sport. Indeed, with the world’s top players all competing in Melbourne, there has been plenty of material to keep alive a story that has now hit the player right at the top of the men’s game.

On the eve of the tournament in Melbourne, an investigation from BBC and Buzzfeed alleged that 16 top-50 players, including Grand Slam winners in both singles and doubles, had been repeatedly flagged for match-fixing in the past decade, but faced no sanction. According to the report, the Tennis Integrity Unit, set up to monitor betting patterns, has been toothless in investigating the evidence.

The head of the Association of Tennis Professionals [ATP], who run the men’s tour, Chris Kermode, is among those to dismiss the seriousness of the problem and the suggestion of negligence on the part of the investigators. It is widely suspected that the problem is concentrated lower than the sport, where the prize money can often leave players struggling to survive on the tour. Yet even the world No. 1 and favorite to claim the Australian Open title, Novak Djokovic, has been accused of wrongdoing.

Earlier this week, Djokovic repeated a story that he was offered $200,000 to lose a match in 2007. The Serbian, though, insisted that “Of course, we threw it away right away. It didn’t even get to me."

Potentially more concerning, is a claim from Italian publication Tuttosport that he had deliberately lost a match to now-retired French player Fabrice Santoro at the prestigious Masters 1000 event in Paris in 2007. Djokovic, who has won 10 Grand Slam titles, was repeatedly questioned about the allegation after his second-round victory over Quentin Halys on Wednesday.

“It's not true,” Djokovic said after being pressed. “I have nothing more to say, guys. If you have any other questions on any other subject, I'm ready to talk about this. I have nothing more to say.”

“What it is to say? I've lost that match,” he said earlier in the press conference. “I don't know if you're trying to create a story about that match or for that matter any of the matches of the top players losing in the early rounds, I think it's just absurd.

“Anybody can create a story about any match. You can pick any match that you like that the top player lost and just create a story out of it. I think it's not supported by any kind of proof, any evidence, any facts. It's just speculation. So I don't think there is a story about it.”

The investigation by the BBC and Buzzfeed did not provide the names of those players suspected, however. Roger Federer, winner of a record 17 Grand Slam titles, has stressed that more specifics are needed.

“I mean, it's like who, what. It's like thrown around. It's so easy to do that," Federer said. "I would love to hear names. Then at least it's concrete stuff and you can actually debate about it. Was it the player? Was it the support team? Who was it? Was it before? Was it a doubles player, a singles player? Which slam? It's so all over the place. It's nonsense to answer something that is pure speculation.”

Still, former Wimbledon and U.S. Open champion Andy Murray, who notably retweeted the Buzzfeed report, has stated that the tennis authorities could be more open about the issue.

“As a player you just want to be made aware kind of everything that's going on,” he said on Tuesday. “I think we deserve to know everything that's sort of out there. Some of it will be true; some of it might not be true. But I'm always very curious with that stuff across really all sports, as well. I think sports could in general be much, much more transparent.”