The former '30 Rock' star took to the stand against Genevieve Sabourin, who is on trial for stalking and harassment, in Manhattan on Tuesday morning, and broke down in tears as he accused the Canadian actress of making his life miserable.
The defendant, 40, repeatedly heckled the 'Rock of Ages' star, 55, during his testimony, according to gossip website TMZ.com, and challenged Alec when he denied her claims that they had sex after a dinner date in 2010 and also met in 2002.
She shouted: "Really?" when he insisted they never had a sexual relationship and accused him of having a "lying disease."
Prosecutors revealed last week that Genevieve, who was arrested outside Alec's home in New York City in April, violating a restraining order, wrote him more than 100 emails in a bid to convince him to get her pregnant, and even told him when she would be ovulating.
The actress's lawyer, Todd Spodek, argued that her actions were the result of her desire to seek closure after her brief "sexual relationship" with Alec "crumbled" in 2010.
He previously said: "No contact and no decency. A complete disregard for Miss Sabourin and her emotions. Miss Sabourin was just looking for closure from a difficult relationship."
Alec's wife, Hilaria Thomas, 29, who welcomed their first child, Carmen, in August, is expected to testify later this week.