Good Morning Star pipped favourite Betterbetterbetter in the Cheshire Oaks at Chester.
The 50/1 shot, trained by Mark Johnston and ridden by Franny Norton, had plenty to find at the weights but stayed on very strongly to get up on the line.
The well-fancied Betterbetterbetter (2/1 favourite) looked to have the race in the bag a furlong from home but she tied up late on under Joseph O'Brien.
In a great result for bookmakers, the other well-supported runner, Ralph Beckett's Cubanita (5/2) finished third.
Koukash said: "Last year I had to wait until the second day to have a winner so it's great.
"She was my only social runner of the meeting. I rang Mark up and suggested we ran her as I had nothing else that would stay a mile and a half.
"Franny gave her a fantastic ride and I turned to my friend I was watching the race with and said she didn't look like a 50-1 chance.
"I had been looking for a claimer for her but we'll probably take her to Royal Ascot now over a mile and a half (Ribblesdale) and see what happens.
"She's a very valuable filly now and we'll keep her to breed off."
Johnston said: "We'll have to go home now and take stock (with regard to plans).
"If it wasn't for Dr Koukash she wouldn't have been running here. I think he would rather win this than the Epsom Oaks and she was the best we had.
"We took our chance and it was an inspired decision."
O'Brien said of the runner-up: "I was delighted with her run. She's green and lazy at home and she would have learned a lot from today.
"She was just a little idle when she got there, but she's run a great race and Joseph was delighted with her.
"We brought her here to find out if she was an Oaks filly and on what I have seen, I couldn't say she isn't."
Ralph Beckett said of Cubanita: "It was a good run but she was awkward round those bends and she just didn't look happy.
"Importantly she picked up some black type and any day a filly does that is a good day."