A CYCLIST who rode into a Mercedes Benz early on New Year's Day responded by bashing the driver and stealing the luxury convertible, police say.

The cyclist was riding along a footpath near the intersection of Morphett St and Andrew St in Adelaide about 1am when he rode into the passenger side of a black 2007 Mercedes convertible that was stopped at a give way sign.

Police said the driver exited his car to check on the cyclist, who reacted by assaulting him and stealing the luxury car.

Witness Dominic Carrocci was walking along Morphett St with his wife when they heard the impact of the collision.

"The cyclist went over the bonnet and his bike was stuck under the car and the cyclist was yelling 'reverse up, reverse up' and we heard a bit more commotion," Mr Carrocci said.

"In a matter of seconds he snapped and attacked the driver, he punched him four or five times in the face and kicked him once before driving off."

Mr Carrocci said the driver suffered a cut mouth, swollen cheek and was shaken by the incident.

Compounding the driver's angst was the fact that the convertible had been borrowed from a friend.

"It wasn't his car, it was his friend's so he wasn't very happy about that either," he said.

The car was found abandoned in Turnbull Place at Brompton about 2.30pm on Wednesday.

Police said the cyclist was of Caucasian appearance, in his late 20s, between 172-174cm tall with a slim build, short or balding hair and blue eyes.

Police are also hoping to speak with a female pedestrian who witnessed the incident but left before patrols arrived.

The bicycle - which police believe was itself stolen - is a distinctive black and white Merida brand with a crossbar bag and broken rear light.

Anyone who witnessed the incident or has information should contact Crime Stoppers on 1800 333 000.