Improving filly Spanish Reef has outgunned favourite Ruby Sea to win the $120,000 Rivette Series Final at Flemington.

The Ken Keys-trained Spanish Reef went into Saturday's 1600m race for fillies off a last-start win at Sandown in a heat of the series and backed it up with another tough performance.

The Ciaron Maher-trained Ruby Sea ($2.60) came from back in the field and made ground to loom strongly but Spanish Reef ($7.50) had a late kick and got home by three-quarters of a length.

Summer Glen ($19) was another length away in third.

"She has got a real lot of ability but has been a bit slow to mature in the brain," Keys said.

"It's starting to come together now. And on what I saw today, and what she's bred to do, I think she will be a good mare over a mile-and-a half."

Keys said he would look to continue Spanish Reef's campaign for another run over 2000m at the end of the season before she turns four.