Outside court, attorney for the Townsend family Ben Crump echoed the song when asked how he felt the day would go, telling reporters, ""Like Marvin Gaye's saying, 'Let's Get It On.'"
The trial is the first of three Sheeran could face from lawsuits over similarities between the two hits.
Marvin Gaye collaborated with Townsend to write "Let's Get It On," which topped the Billboard charts in 1973. Sheeran's "Thinking Out Loud" peaked at No. 2 on the Billboard Hot 100 in 2015.
Townsend's heirs sued Sheeran for copyright infringement in 2017, claiming "Thinking Out Loud" copied the "heart" of Gaye's song including its melody, harmony and rhythm.
The lawsuit said Sheeran has performed the two songs live as a medley and transitioned "seamlessly" between them. Sheeran has argued that any similarities between the songs involve basic musical "building blocks" that are ineligible for copyright protection.
