Since the start of the year 30 professional matches from the 24,300 played have aroused suspicion -- a drop from 48 during the same period in 2016.

"Having fully assessed each of these (three) cases, no evidence of corrupt activity was identified by the TIU," a statement said.

An alert, according to the TIU, is merely an indicator of something suspicious such as an abnormal betting pattern.

Seventeen of the alerts so far this year have come on the lowest level ITF tournaments.

Anti-corruption procedures came into the spotlight at the 2016 Australian Open after a BuzzFeed News investigation claimed there was widespread match-fixing at the top of the men's game.

The claims were denied by the TIU and tennis authorities and no high-level players have been charged with wrongdoing.

The TIU has beefed up its anti-corruption measures since then and in March introduced a new online training portal designed to educate players against match-fixing, grooming, betting on matches and illegal approaches.

So far 3,500 players have completed the new programme.

(Reporting by Martyn Herman)