Champion filly Songbird has kept her unbeaten record intact with a dominant win in the American Oaks at Saratoga Springs in New York.

The highest-ranked two-year-old in the world last year, Songbird took her record to nine from nine with a 5-1/4 length win over Carina Mia in the Group One race over 1800 metres.

Carina Mia stalked Songbird until the 400 metres but could not respond when Songbird accelerated under Mike Smith.

"I knew at some point Carina Mia was going to give her a serious test, an acid test, and she did," Smith told the Daily Racing Form.

"She made her do something she's never had to do before.

"A lot of times, a true champion, this will move them up because it's the first time somebody got inside her gut. If that happens, oh my."

Songbird will stay in Saratoga and be aimed at the Group One Alabama on August 20.