NEW YORK Watch what you tweet.

Thats the message tennis authorities are delivering as the US Open gets set to start on Monday, telling players and their entourages to be careful about what they post on the social networking site Twitter.

Signs are being posted in the players lounge, locker rooms and referees office at the USTA Billie Jean King National Tennis Center with the header 'Important. Player Notice. Twitter Warning.'

The signs, written by the Tennis Integrity Unit, point out that Twitter messages could violate the sports anti-corruption rules.

'Many of you will have Twitter accounts in order for your fans to follow you and to become more engaged in you and the sport _ and this is great,' the notices read.

'However popular it is, it is important to warn you of some of the dangers posted by Twittering as it relates to the Tennis Anti-Corruption Program Rules.'

Sports leagues and governing bodies are paying close attention as more and more athletes turn to Twitter to reach fans directly; some NFL teams, for example, urged players not to use it. But tennis appears to be the first sport openly concerned about Twitters possible effect on gambling.

The signs at the US Open say tweeting is not allowed on court during matches. They also warn about using Twitter away from the court, saying sending 'certain sensitive information concerning your match or other matches and/or players should be avoided. Depending on the information sent out this could be determined as the passing of inside information.'

The messages define that as 'information about the likely participation or likely performance of a player in an event or concerning the weather, court conditions, status, outcome or any other aspect of an event which is known by a Covered Person and is not information in the public domain.'

The warnings say they apply to players, coaches, agents, family members and tournament staff.

'We take our anti-gambling procedures very seriously, and were in full agreement with this recommendation from the Tennis Integrity Unit,' US Open spokesman Chris Widmaier said on Friday.

Twitter, launched in 2006, first gained popularity as a way for fans to follow the thoughts and activities of celebrities via messages of 140 characters or fewer.

Several prominent tennis players are part of the trend, including defending US Open champion Serena Williams (who has nearly 1 million followers) and 2003 winner Andy Roddick (more than 100,000).

Roddick wrote on Friday that he 'snuck in a hit indoors today ... .met will ferrell out at the site today!!'

Athletes post everything from personal blog links to updates on their social lives to injury updates. That last category is the sort of thing that worries the world of tennis, which revamped its anti-corruption policing a year ago in the wake of an investigation into match-fixing.

Jeff Rees, whose name appears at the bottom of the signs posted at the US Open, was appointed in August 2008 to run the Tennis Integrity Unit. Thats also when tennis four governing bodies _ the ATP and WTA tours, the

International Tennis Federation and the Grand Slam Committee _ adopted an anti-corruption code to make sure the same rules and penalties are applied across the sport.