Investigators are trying to trace the internet punters who triggered a £3.5 million flurry of betting on an obscure match and raised fears that the sport could be at the mercy of match-fixing.

Betfair, the internet exchange, declared all bets void on the match between Nikolay Davydenko, the world No 4, and Martin Vassallo Arguello, an Argentinian ranked 83 places lower, in the Orange Prokom Open in Poland, and handed details of suspicious betting patterns to the Association of Tennis Professionals (ATP).

Betfair investigators were alerted after Davydenko, the tournament favourite in Sopot, cruised through the first set on Thursday, winning it 6-2, only for the money to follow Arguello before the start of the second. The Argentinian won the second set before Davydenko retired with a foot injury while 2-1 down in the third. By the time Betfair closed the book, more than £3.5 million in stake money had built up on a contest that hardly rated headlines.

As Betfair set about checking whom the money had come from, the company’s internet forum was full of messages from punters suspicious that a match-fixing ring was at work, using the cover of low-profile tournaments. It is a rumour that has hung over tennis for four years.

But controversy landed on the ATP’s doormat with a thump yesterday with the news of Betfair’s decision to cancel betting on the Davydenko match. Betfair refused to confirm that they were worried about match-fixing but said they were concerned only with the strange betting patterns that had money being splashed on a contest between what should have been a no-hoper and a tournament favourite who was expected to walk away with the minor title.

Mark Davies, the managing director of Betfair, said: “What worried us was that the price on Davydenko started to move before his injury became apparent. We did the only thing we could do and declared all bets void.”

The ATP struck a deal with Betfair in 2003 that any suspicious betting or single bets of more than £100,000 would be reported to the governing body in an effort to head off accusations of match-fixing. So far, incidents have been few and far between, although Wimbledon was briefly overshadowed last year when Richard Bloomfield, a British wildcard, beat Carlos Berlocq, who was 170 places above him in the world rankings. It was Betfair, again, who reported that more than £300,000 had been bet on the match.