Corruption in tennis is again under the microscope following the withdrawal, in questionable of circumstances, of women's world No.6 Caroline Wozniacki from a match at the Luxembourg Open last week.

Wozniacki was leading Luxembourg's Anne Kremer, 7-5, 5-0, in the first-round match when she retired, citing a leg injury. Such were the unusual circumstances surrounding the withdrawal that the Women's Tennis Association has launched an investigation into the incident.

Wozniacki was known to be carrying a leg injury and later explained that, because she knew she would not be able to fulfil her second-round tie, "chose the sporting option" and allowed Kremer to go through.

But following complaints and posted on internet gambling websites, the WTA launched an investigation into the matter and might well pass it on to tennis's anti-corruption unit.

Wozniacki, and her retinue have denied any wrongdoing. "Caroline had an injury to her hamstring and back, and because she was not going to be able to continue to the next round she wanted to be a good sport and allow the other girl to continue playing," her management company said in a statement. "We understand how it looks, but we would not want this to be blown out of proportion."

That is a plausible explanation - kind of. But why did she insist on taking the court in the first place if she was carrying the injury? Why get to 5-0 in the second set, and within a handful of points from claiming the match, before retiring? Why not take the ranking points, and prizemoney, then forfeit the second-round tie? The WTA investigators will no doubt be asking the same questions.

Match-fixing, or the suspicion of match-fixing, is a growing blight on tennis, and an issue that increasingly exercises the minds of tennis officialdom. The Tennis Inegrity Unit was introduced two years ago to try and break the nexus between players and the massive gambling syndicates that prey on the lowly-ranked, easily led and impecunious among them.

Professional punters who follow the betting trends before, and during, ATP and WTA matches will sometimes spot alarming disparities between what the odds on a particular match should be, and what they are. For example, there might be a flood of money on an underdog for no apparent reason and then that underdog will defy his or her lowly ranking to score an upset win.

Suspicious results abound, but there is rarely any proof that they are anything but common or garden sporting upsets. And, as we know, those upsets happen all the time in sport. So it's very hard to discern between a genuine win-against-the-odds and a crooked, contrived result. This is the problem facing the anti-corruption unit: unless incriminating phone conversations are tapped, or questionable bank deposits are traced back to shady bookmakers or betting syndicates, it will remain very hard for the corruption-busters to secure a conviction.

A couple of weeks ago, Russia's Nikolai Davydenko completed one of the best fortnights of his career. He won a couple of events, including the Shanghai Masters, where he defeated Novak Djokovic in a semi-final and Rafael Nadal in the final.

Fresh from that triumph, Davydenko arrived in Moscow last week for the Kremlin Cup, an event he had won three times previously. On form, and in the absence of many of the top-liners, he was installed as top seed and a short-priced favourite.

But he was drawn to meet his fellow-countryman Marat Safin in the first round. That match-up immediately made some punters uneasy. Safin had been in woeful form for much of the season but was due to retire from the game in a month. This meant he would probably be appearing in his hometown of Moscow, where he'd always been a local favourite, for the last time. On top of that, he and Davydenko were Davis Cup teammates and Davydenko was guaranteed a berth in next month's ATP World Tour Finals in London regardless of whether he won or lost his first-round match. Alarm bells started ringing all over the place.

On form alone, Safin should legitimately have been a 6/1 or 7/1 shot. But he was installed as a 9/4 underdog. One punter outlaid $25,000 on Safin at that price, and the money kept coming for him.

The result? Well, put it this way: not many punters were hugely surprised to see Safin win, 4-6 6-4 6-2. Davydenko later claimed he was tired after his strenuous fortnight, and the flight back from Shanghai.

There is nothing to suggest this result was anything other than legitimate. But if you were a cynical, suspicious, conspiracy-theorist type, you might be tempted to think otherwise. This is the depth of the problem facing the Integrity Unit and, by extension, the entire sport of professional tennis.