The Robert Smerdon trained filly was there to be beaten over the final furlong in the 2000 metre staying test, but refused to lie down, staying on with grim determination to hold out Crafty and win by a neck for Mark Zahra.
"She's a great horse," said Zahra.
"She's so tough and just keeps sticking her head out and fighting. You'd like a stable full of them."
A last start winner of the Group II VRC Edward Manifold Stakes, Fontein Ruby has the overall record of four wins and three placings from 10 starts with prizemoney topping $743,000.
"She has a great constitution and just keeps pulling something out," said Smerdon, who also prepares the runner-up Crafty, with both fillies to press on to the Oaks all going well.
A $50,000 Inglis Premier purchase for Aquanita Racing from the draft of Blue Gum Farm, Fontein Ruby runs for a big group of owners and is from the unraced Rubiton mare Charming Ruby, a three-quarter sister-in-blood to dual Cox Plate hero Fields of Omagh.
Bred by Blue Gum Farm in partnership, she was a Foal Gallery star in 2011 and is pictured here with her dam at Blue Gum Farm.
Charming Ruby was sold for just $17,000 at the 2012 Inglis Great Southern Sale and has produced a two year-old filly by Turffontein and yearling colt by Master of Design for her new owners.
She was covered again by Master of Design last spring.
Fontein Ruby is the best performer to date for Turffontein, who stands at Blue Gum Farm at a fee of $7,700.