Fontiton has shown why she has alway been at the top of Blue Diamond betting with a convincing win in the fillies Prelude at Caulfield.

The Robert Smerdon-trained filly faced her biggest test in her third start in Saturday's Group Two race but after being under pressure turning for home she responded to stamp her authority.

Fontiton, well backed to start the $2.50 favourite, settled fourth behind a strong speed but once she balanced up and jockey Mark Zahra asked her to respond, she went on to score by 1-3/4 lengths.

Miss Gidget finished second with Flamboyant Lass taking third.

The victory confirmed Fontiton as the horse to beat in the $1 million Blue Diamond Stakes on February 28.

"The way she won was like she was looking for the 1200 metres," Zahra said.

"I came off the bit at the top of the straight so going to a Blue Diamond, I couldn't be happier.

"As soon as I got fair dinkum and she straightened up, she found."

Fontiton will try to emulate Samaready (2012) and Earthquake (2014) as a Blue Diamond-winning filly after a Prelude victory.

"I was impressed by the way she went through the line because when she straightened she had to be put under pressure and she had to fight," Smerdon said.

"She did that and she found the line."

Second favourite Secret Agenda ($4.20) finished 11th after getting shuffled back.

Damien Oliver said it was a rough race and the filly met with interference from the 900m to the home turn.

"We never got a chance to see how she goes," Oliver said.

Stewards reported she suffered a laceration to her near-fore cannon bone.

Antelucan was eased down to finish last, with Chad Schofield saying she found trouble at the 800m.