The International Tennis Federation, the ATP, the WTA Tour and the Grand Slam Committee met Friday to discuss the issue, three days after 18th-ranked Andy Murray said fixed matches are common knowledge.

"While we do not believe that our sport has a corruption problem, we do recognize that a threat to the integrity of tennis exists," the group said in a statement.

Roger Federer said he was surprised to hear match-fixing may be happening.

"I've been around for the last 10 years and have never been approached or ever heard anything about it until these comments came out," the 12-time Grand Slam champion said in Madrid, Spain.

Murray, who was playing at the Kremlin Cup in Moscow this week, has already backed off his claim in a statement on his Web site. He wanted to clarify remarks he felt were taken out of context.

"When I said 'everyone knows that it's going on,' I meant that everyone has probably heard that three or four players have spoken out about being offered money to lose matches — which they refused," he said.

Tennis officials have been discussing ways to keep the sport free of match-fixing, and they have sought help from experts in other sports.

"We believe that an independent situation analysis of this risk is necessary." the group said.

Murray's comments earlier this week led the ATP to ask him to explain himself at a meeting. That is expected to happen Monday at the Madrid Masters.

Ivan Ljubicic, the president of the ATP Player Council who is playing at the BA-CA Tennis Trophy in Austria, became the third player to question Murray's remarks.

"I think Andy Murray talked more about what he heard than about what he knows," Ljubicic said Friday after losing in the quarterfinals of the tournament in Vienna. "I am absolutely sure that none of the top players is involved in betting or match-fixing."

On Thursday, Nikolay Davydenko, the player at the center of the sport's biggest match-fixing scandal, criticized Murray by saying if the British player knew it was going then he must be involved himself. Second-ranked Rafael Nadal also expressed doubts about Murray's claims.

Match-fixing rumors have swirled around tennis the last few months. An online betting site, in an unprecedented move, voided bets on a match in August because of irregular betting patterns. Davydenko withdrew from that match in Poland against 87th-ranked Martin Vassallo Arguello in the third set because of a foot injury, and the ATP is looking into it.

"We all know what happened with Davydenko in Poland," Ljubicic said. "But I talked to his trainer and he said that Davydenko really had a problem and even received medical treatment before that match. So we can't draw any conclusions as long as nothing has been proven."

Tennis officials have also received a document that lists professional matches considered to be suspicious, including some at Grand Slams, dating to 2002, and Murray's original comments revived talks about a rule requiring players to tell the ATP within two days any information they may have regarding match-fixing.

"It was not good what Andy did. If you know something, you have to go to the ATP immediately, not wait 10 months and then speak out about what happened all that time ago," Ljubicic said.

"The problem are matches against low-ranked players," the Croat added. "I think betting agencies should not offer these matches in the first place as there could always be some kind of risk due to the high stakes involved in such matches."

Since the Davydenko match, others have said they have been approached by outsiders trying to influence a match. Last month, Belgian player Gilles Elseneer said he was offered — and turned down — more than $100,000 to lose a first-round match against Potito Starace of Italy at Wimbledon in 2005.

"We have to do everything we can from our side to take that out from our sport, to keep our sport clean," said Mario Ancic, who is playing at the Stockholm Open.

On the women's tour, a match in September drew suspicion for unusual betting patterns.

An online betting site briefly delayed payment after 120th-ranked Mariya Koryttseva beat No. 96 Tatiana Poutchek in the quarterfinals of a tournament in India. Eventually, bets were paid, and both the WTA and the betting site said they doubt there was any wrongdoing connected to the match.

"I've obviously heard about it but haven't really been following the whole match-fixing thing," said Serena Williams, who reached the Kremlin Cup semifinals Friday. "In America, you are innocent until your proven guilty. That's how it is."