A former adviser to Malcolm Turnbull who was challenging Tony Abbott in his blue-ribbon Sydney seat of Warringah has pulled out of the contest and thrown her support behind a campaign by world champion athlete Zali Steggall to take down the former prime minister.

In a move designed to maximise the chances of defeating Mr Abbott at the May election, Alice Thompson told The Sydney Morning Herald and The Age she would withdraw from the battle and move her campaign to the adjoining northern beaches seat of Mackellar.

"There is no point in undercutting Zali here, I want all my volunteers and supporters in Warringah to get behind her. She's got this," Ms Thompson said.

Alice Thompson with Zali Steggall at Dee Why Beach. Credit:Christopher Pearce

Ms Steggall, a four-time Winter Olympian-turned barrister who last Sunday announced she would attempt to topple the former prime minister, said her rival's decision to pull out and endorse her was "a win for our campaign".