The Bulldogs have suffered a body blow before Sunday's must-win NRL semi-final against Melbourne when star pivot Braith Anasta was ruled out after failing to overcome a sternum injury.

Anasta flunked a fitness test on Saturday morning, ensuring 18th man Johnathan Thurston will step up to play five-eighth at Aussie Stadium on Sunday.

Thurston will start and will share pivot duties with reserve Reni Maitua.

Anasta suffered sternum bruising during the Bulldogs' shock qualifying final loss to the North Queensland Cowboys last Saturday night.

Anasta struggled during the game but looked set to play in the crucial clash against Melbourne after showing improvement at training this week.

However, Anasta couldn't complete his first contact session of the week on Saturday morning, opening the door for youngster Thurston.

"He trained during the week but he was unable to take any contact at all. That makes it a bit difficult in a game of rugby league," Bulldogs football manager Brad Clyde said.

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