Starting an odds-on favourite off the back of an easy win prior, the colt looked in a world of pain at the top of the straight but refused to give up and held off a wall of challengers to prevail narrowly over Spin (I Am Invincible x Roulettes) and Legend Of Condor (Not A Single Doubt x Rhodamine).
Despite returning slower overall and sectional times to the subsequent winner of the fillies division in the next race, Waterhouse was her usual enthusiastic self when assessing her charge for the future.
"This is a serious colt and I'll tell you why he's a serious colt, because things were against him today, they really were," she said.
"I thought it was interesting what Tim (Clark – jockey) said about the wind. It was blowing quite strongly, and the ground is a bit soft so you can't use your turn of foot quite as well.
"But I really think he's a serious chance in the Slipper, he's a horse that has got a beautiful action."
Santos was a $180,000 yearling purchase for the Waterhouse/Bott partnership from the Emirates Stud draft at the Inglis Classic Yearling Sale.
He is one of five winners from the city winning Secret Savings mare Ghaliah who has also left the Japanese stakes winner Melagrana (Fastnet Rock).
She is a half-sister to four stakeswinners including Mutawaajid, Adeewin, Fatoon and Jivago but sadly died five days after giving birth to her 2016 foal by Smart Missile
It is the 13 stakes winner of the season for I Am Invincible who has 12 yearlings in the upcoming Inglis Melbourne Premier Sale and a whopping 41 at Easter.