An umpire who was suspended as part of a betting investigation worked at last year's US Open, the United States Tennis Association (USTA) confirmed.
Croatia's Denis Pitner was banned for a year in August after passing on details about a player's fitness and accessing an account used to place bets.
Pitner was approved by the US Open on 13 July and picked up his credential before the USTA was notified of a ban.
The USTA responded on Friday, blaming "a flaw in our process".
A statement said: "After learning within the last 24 hours that an official on the 'Do Not Credential' list may have worked at the 2015 US Open as a linesman, the USTA immediately investigated the claim.