An online bookmaker last night suspended payments on a tennis match involving world No 4 Nikolay Davydenko due to irregular betting patterns.

Betfair launched an investigation following the retirement of the Russian during a match against Martin Vassallo, the world No 87, at the Poland Open in Sopot.

Davydenko won the first set 6-2 but then lost the second 6-3 and was trailing 2-1 in the third when he withdrew with a foot injury.

Betfair reported an usually high number of bets on the match and are to inform the ATP Tour, the governing body, of their concerns. About £3.4 million had been bet before the match and during the first set, ten times the amount typical for a match of this kind.

"The ATP takes issues surrounding gambling extremely seriously," a spokesman for the tour said. "We are committed to ensuring our sport remains corruption free and have strict rules in place governing this area.

"It would be inappropriate for us to comment further on any individual match or on the status of any potential investigation until such time as the process has been completed."

Davydenko, the defending champion in Sopot, has now lost in the first round of his last three tournaments. "I had problems with my foot in the first round against Andrei Pavel," Davydenko, who will be amongst the top seeds at the US Open which starts on August 27, said. "I have to think about other important tournaments. I don't want to aggravate the injury."