The TIU received an alert regarding one match played at the grass-court Grand Slam in London.
Its alerts come from regulators and betting organisations, who can report matches if they notice suspicious betting patterns.
The TIU said it received 96 alerts from July to September, of which two came in Grand Slams, the first at Wimbledon and the second at the US Open.
A TIU statement said: "Historically, grand slams receive very few match alerts and in keeping with that record, only two were received during the period; one at Wimbledon, the other at the US Open.
"Both are the subject of routine, confidential investigation by the TIU."
The US Open match under investigation had already been disclosed, with the TIU having announced in September it was looking at the match between Vitalia Diatchenko and Timea Bacsinszky. It has not given details of the Wimbledon match that is under scrutiny.
The TIU's guidance states: "It is important to appreciate that an alert on its own is not evidence of match-fixing."