Former British number one Andrew Castle said on Monday 18 January that he could see how there could be a temptation to match-fix in lower ranked tennis tournaments for some players. Big financial incentives for gamblers, small tournaments of little consequence to top players and the ease of fixing the outcome of a one-on-one sport have helped turn world tennis into a haven for match-fixers, according to experts and industry insiders.

Tennis was rocked by allegations that the game's authorities have failed to deal with widespread match-fixing, just as the Australian Open, the first grand slam tournament of the year, kicked off in Melbourne. "This is online gambling, already people have been done for this in the past and now a full and thorough investigation needs to take place, it is as simple as that," Castle, now a tennis commentator, said.

"We don't know whether it's male or female, we don't know when it was, whether it was singles, doubles or mixed. So again this sort of speculation is unhelpful to the game. How concerned am I? Very. It's one thing in a Challenger somewhere, so many matches get bet on, it's quite another with a hugely high profile match at one of the biggest tournaments in the world. So yes there are different categories and it does worry me.

"The matches I watch I don't see that but let me tell you, at the lower end there is temptation, of course there is. If you're making £30,000 or £40,000 ($43,000 to $57,000) a year and somebody comes up to you and says, I don't know, for instance, double fault on the third point of your second service game and I'm going to double your yearly wages if you do that, that is one scenario of many."

Castle said he did not think there was a problem at the top of the game. "Nadal, [Novak] Djokovic, [Andy] Murray, [Roger] Federer, [Stan] Wawrinka, all that lot I think they're immune to it because I think there's a vast amount of money up there the temptation is not there. Lower down the game we might have a bit of a problem that needs to be looked at, but nothing's being suppressed.

"I have complete faith in the governing body because everyone has a vested interest," Castle added. "If you can't trust what you're watching then it's not worth watching. We don't want to be going down the road of other sports that have been either slow to act or have not acted, and I don't think tennis is in that category at all. So I'm pleased with the response so far but it's got to be robust."

Sports betting has exploded in recent years with the proliferation of online and mobile gambling sites, and could be worth as much as $3tn (£2.1tn) annually, Patrick Jay, an independent betting expert, told the United Nations in 2015. In that sea of money, tennis is the second most active betting market, trailing only football, research conducted by the European Gaming and Betting Association shows.