US Open organisers have admitted a banned umpire was allowed to work as a linesman during last year's tournament.
The United States Tennis Association (USTA) has launched an investigation into why Croatian Denis Pitner, who was handed a 12-month ban for betting offences by the International Tennis Federation, was not prevented from being involved at Flushing Meadows in New York.
The Guardian reported Pitner was put on a banned list on Aug. 24 2015, a day before a four-day qualifying event for the two-week US Open began, but had already collected his accreditation and stayed involved during the tournament itself until Sep. 10.
It also said he officiated at an ATP tournament in Doha last month, and at a veterans' event in Croatia while banned. The ITF previously said Pitner had his certification suspended on Aug. 1.
Pitner was handed his ban for passing on fitness details of a player to a coach and accessing a betting account from which tennis bets were placed.
The USTA said in a statement: "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. The USTA was shocked to find that this was in fact the case.
"As we have now determined, Denis Pitner had been approved to work the US Open as a linesman on 13 July. The USTA was notified that Mr Pitner was placed on the 'do not credential' list on 24 August. Mr Pitner had already picked up his credential prior to the USTA being notified. Due to a flaw in our process, which we are investigating now, Mr Pitner's credential was not cancelled. For this reason, he did work as a linesman at the 2015 US Open."
In its statement to the Guardian, the USTA said: "The USTA takes this matter extremely seriously and has made the investigation of what caused the error its highest priority. We will also work with the newly created independent review panel to ensure instances such as this are not repeated in the future."
It emerged on Tuesday that Pitner was one of two officials banned by the ITF. Kirill Parfenov of Kazakhstan was banned for life in February 2015, after making contact with another official on social media in a bid to manipulate match scoring.
ITF president David Haggerty told the New York Times that Pitner's involvement at the US Open was probably caused by an innocent breakdown in communication.
He said: "It sounds like he got his credential before anybody realised what had happened. An innocent mistake probably happens there, but I think what we want to do is make sure going forward that we are all communicating, and everybody is aware of what is going on."
The Tennis Integrity Unit and ITF explained the bans for Parfenov and Pitner were not made public because the ITF code of conduct did not allow for punishments to be announced.
That code was amended in December 2015 to allow for sanctions to be revealed.