The 34-year-old was watching her friend, prosecutor Leigh Bishop, in a case against a father accused of shaking to death his baby daughter.
Li Hangbin, 28, allegedly killed two-month-old Annie in 2007 and has been in custody for five years awaiting trial, reported Sun Online.
Holmes, 34, sat in the audience to watch the opening statement but reportedly left after a brief time.
The actress is said to have a lot of time on hands right now after producers pulled the plug on her Broadway play after just six weeks.
Terrible ticket sales meant Dead Accounts was axed after weeks of playing to tiny audiences.
Holmes, whose appearance in the play was meant to be her big acting comeback following her split from Tom Cruise, failed to draw in big audiences and the final curtain fell on January 6.