An ex-tennis player from South America has told the BBC that match-fixing is commonplace and even some elite players are "a little bit dirty in some way".
He also claimed fixing is not just limited to lower-ranked professionals and is "a secret that everybody knows".
The player, who requested anonymity, said tennis authorities "know who is doing it" but are unwilling to stop it.
The Tennis Integrity Unit (TIU) said it rejects "any suggestion that evidence of match-fixing has been suppressed".
"We invite the player behind the allegations to make contact with the TIU and to share the information he claims to have," the TIU added in a statement.
The allegations come after a BBC and BuzzFeed News investigation revealed suspected illegal betting in tennis over the past decade.
In an exclusive interview with the BBC's World Have Your Say team, the player, who featured in several tour matches last year and is now a coach, detailed how the fixers operate and the lengths they go to in order to remain undetected.
"This is like a secret on the tour that everybody knows, but we don't talk about it," said the player, whose identity is known by the BBC. "We just see it and keep working."
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How are matches fixed?
The player claimed "three big groups" control betting in tennis and that any payments to players are made using cash, with no bank-to-bank transfers allowed.
"Each group has many guys who go to talk to players," he said. "They have many guys inside the circuit.
"Also, they have many accounts. They have 50-60 accounts where they place small money. At the end, it's huge money. It's really big."
The BBC subsequently attempted to contact the player again to ask for clarification on exactly how much a player could earn from match-fixing in a year, but he was unavailable.
How do you know who is involved?
"You know who is doing it and who is not," he continued. "As a player, I know who is missing on purpose or returning a shot in the middle on purpose... who is trying, and who is not. So we work on this. We know."
He also claimed players exchange knowing smiles and make comments that indicate they have fixed a match.
"I started to believe [top players were involved] a few years ago, when a guy told me the result of the next two tournaments" he said.
"He told me exactly who was going to win and how it was going to happen.
"In the beginning, I thought he was just bragging about it to make me fall for his game. But then I was laughing that every match was happening the way he had been telling me it was going to happen... and I'm talking about a Masters series, where there are just big names."
So you were told who was going to win?
Not just that, added the player, but "exactly" how they would win.
"When I was watching it myself," he said, "I couldn't believe it. It's not easy knowing that you have to lose. You start hitting it and, trust me, everything goes in… it can make you panic.
"So when I see the guy winning so easily and then he's missing absolutely on purpose, every ball, and the other guy wins... I just couldn't believe it."
Why not go to the authorities?
"We could co-operate with Tennis Integrity if we wanted to, but they don't want it to be stopped," he said.
He claimed fewer players would be tempted to fix if they were getting paid more, insisting a player ranked 400 in the world cannot make a living out of tennis.
"They [the authorities] know exactly who is doing it and, if they wanted to stop it, they could stop it today. It's super-easy. They just don't want to do it."
In response, the TIU said it has a "zero-tolerance approach which is enforced with the full powers of the Tennis Anti-Corruption Program that includes lifetime bans and punitive financial penalties".
It added: "The TIU works closely with players to prevent corruption through education programmes and confidential reporting systems.
"The great majority of the 21,000 active professional players are good people of high integrity who abhor the suggestion that the sport they love is tainted with allegations of corruption."
What do professional players earn?
A study conducted on behalf of the International Tennis Federation in 2013 showed that 45% of the 13,736 players at all professional levels of the sport earned nothing from it and only about 10% covered their costs.
Of 8,874 male and 4,862 female respondents to the survey, 3,896 male and 2,212 female earned no prize money.
Other findings in the study, conducted by Kingston University and calculated here at the 2016 exchange rate, showed:
- The 2013 'break-even' world ranking for women - where the cost of competing was matched by earnings - was 253. For men, the ranking was 336.
- The top 1% of ranked male players - the top 50 - earned 60% of the £113m total prize money pool.
- In the women's game, the top 50 earned 51% of the total prize money pool of £84m.
- Players ranked in the top 50 earned on average more than £700,000 a year on both the men's and women's tours.
- Those from 51 to 100 earned in excess of £140,000, while 101 to 250 average around £59,500.
- For players ranked from 251 to 500, the earnings were just £11,200 a year.
- The average cost of playing tennis in 2013 - travel, food, accommodation - was £27,100 for men and £28,100 for women.
What are the elite players saying?
British number one Andy Murray said he has never been approached to fix a match and called on the tennis authorities to be "proactive".
"As a player, you just want to be made aware of everything that's going on. I think we deserve to know everything that's out there," he said.
World number one Novak Djokovic says he rejected £110,000 to lose a match early in his career but said there is "no real proof" of fixing among the elite.
"From my knowledge and information about match-fixing, there is nothing happening at the top level, as far as I know," he said.
Seventeen-time Grand Slam champion Roger Federer said he wanted more information about who might be guilty, saying: "I would love to hear names. Then at least it's concrete stuff and you can actually debate about it."
Women's world number one Serena Williams said that if match-fixing was taking place she "didn't know about it", adding: "When I'm playing, I can only answer for me. I play very hard, and every player I play seems to play hard."
Full TIU response to the claims
In a statement released to the BBC, the TIU said: "The TIU and the tennis authorities absolutely reject any suggestion that evidence of match-fixing has been suppressed for any reason.
"The sport has a zero-tolerance approach which is enforced with the full powers of the Tennis Anti-Corruption Program, which includes lifetime bans and punitive financial penalties.
"Since 2009 all professional players, support staff and officials have been subject to this stringent code, which makes it compulsory to report any corrupt approaches or knowledge of suspected corrupt practices to the TIU. Failure to do so is a breach of the Program which can be subject to disciplinary action.
"The TIU works closely with players to prevent corruption through education programmes and confidential reporting systems. The great majority of the 21,000 active professional players are good people of high integrity who abhor the suggestion that the sport they love is tainted with allegations of corruption.
"We invite the player behind the allegations to make contact with the TIU and to share the information he claims to have."
The Association Of Tennis Professionals (ATP) was also contacted for comment but did not respond.
Listen to the full BBC World Have Your Say interview.