Opening statements will unfold Friday in the trial of a Colorado dad charged with his fiancée’s murder — and the star witness is the woman with whom he was having an affair.

Patrick Frazee faces life in prison in the death of Kelsey Berreth, 29, who vanished nearly a year ago on Thanksgiving Day.

Prosecutors say Frazee, 33, beat Berreth, the mother of his child, to death with a baseball bat and then burned her body, which was never found.

Frazee’s mistress, Krystal Lee Kenney, a nurse from Idaho, initially lied to investigators — but later spilled her guts, saying her boyfriend confessed to the sick crime, CNN reported.

She claimed Frazee asked her to kill Berreth three separate times — by poisoning her with a caramel macchiato and by assaulting her a metal pipe and a baseball bat — and that she traveled from Idaho to Colorado but chickened out.

In February, Kenney pleaded guilty to a single count of tampering with evidence in exchange for her testimony and a maximum of three years in prison.

She admitted to helping Frazee by getting rid of Berreth’s cellphone — which authorities pinged near Gooding, Idaho, some 800 miles from her home in Woodland Park, Colorado — and discarding Berreth’s keys and a gun.

Frazee is charged with two counts of first-degree murder for two separate theories surrounding Berreth’s death. He also faces three counts of solicitation to commit murder, two counts of a crime of violence and one count of tampering with a body.

Prosecutors allege Frazee blindfolded Berreth and told her to guess the scents of different candles — and that’s when he bludgeoned her with a baseball bat in her living room.

In a wrongful death lawsuit, they accuse Frazee of being driven to kill over a custody battle over their daughter, who was in the backyard when Berreth was allegedly killed.

The girl is currently being cared for by Berreth’s parents.