The US rapper says the song, used in the 2014 advert by the National Party, was an unlicensed version of Lose Yourself, one of his biggest hits.
But the party's lawyers argue it was not actually Lose Yourself, but a track called Eminem-esque which they bought from a stock music library.
The case began on Monday, with the two tracks played in court.
A lawyer for Eight Mile Style - a publishing group representing the artist - said Lose Yourself was "iconic" and "without doubt the jewel in the crown of Eminem's musical work".
