YouTube channel AirflowNZ captured some of the action on Sunday as Defiant sailed back up the Waitemata at the end of their session.
"American Magics [sic] Defiant spent five hours well out of view near Waiheke Island," the account wrote. "They had a wild ride back to dock in the Harbour with huge wind gusts … testing man & machine."
Both American Magic and Team New Zealand trained on Saturday as preparations ramp up ahead of the opening phase of racing in the Christmas Cup in Auckland on December 17-20.
The Americans were the only team sailing on Sunday.
American Magic's second AC75 Patriot arrived in Auckland last week on one of the world's largest cargo airplanes, but won't be ready to be sailed for several weeks.
Meanwhile, the other two challengers INEOS Team UK and Luna Rossa are working on plans to move to Auckland for the final phase of their buildup.
In the coming weeks, all four syndicates' are expected to officially launch their second boats.