In an unprecedented move, British online gambling company Betfair voided all bets today placed on Thursday's second-round match at the Prokom Open in Sopot between the defending champion and No. 4-ranked Davydenko and No. 87-ranked Martin Vassallo Arguello of Argentina.

Betfair said it received about $7 million in bets on the match — 10 times the usual amount — and most of the money was on Arguello to win, even after Davydenko won the first set 6-2.

Arguello won the second set 6-3 and was leading 2-1 in the third when the Russian retired. Davydenko said he aggravated a left foot injury in the second set. He received medical attention from a tournament trainer before deciding to quit.

Betfair, which has had an agreement with the ATP since 2003 to share information on any irregular betting activity, said it was concerned with the volume of wagers coming in on Arguello from the start.

"We think the market quite clearly wasn't fair," Betfair managing director Mark Davies said. "The prices seemed very odd. As a result, in the interest of fairness and integrity and in consultation with the ATP, we have decided to void the market and return all stakes to (bettors)."

It's the first time the company has taken such a step in any sport. Davies said Betfair would turn over its betting records for the ATP to investigate.

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

"In addition we have memorandums of understanding with U.K. and European betting companies that ensures information pertaining to any ATP Tour match that may look suspicious, based upon gambling patterns, is shared with us immediately."

ATP officials said Friday that Davydenko had left Poland.

"Normally I try to fight to the end but it was very painful and I may have done even more damage by trying to finish the match," Davydenko said Thursday after the match. "Since the beginning of Monday I've had a problem with my left toes. Today that became a problem with my foot."

Since losing in the fourth round at Wimbledon to Marcos Baghdatis, Davydenko lost three straight first-round matches _ to Gael Monfils at the Swiss Open, Florent Serra at the Dutch Open and Gilles Simon at the Croatia Open _ before beating Andrei Pavel, 6-3, 6-4 in the opening round in Poland.

Arguello lost 2-6, 6-4, 6-2 on Friday in the quarterfinals to another Argentine player, Jose Acasuso.

"I saw Davydenko playing very well the first set, and I saw also that he had problems with his feet, and that was true, he was not inventing that, so it's difficult to suspect him," Arguello told The Associated Press by telephone from his hotel room in Sopot.

At Wimbledon in 2006, Betfair reported irregular patterns surrounding a first-round match between British wild card Richard Bloomfield and Carlos Berlocq of Argentina.

Berlocq, who was ranked 170 places higher than Bloomfield, lost 6-1, 6-2, 6-2. Most of the bets placed were on Berlocq to lose. However, no wrongdoing was detected.