Fledgling country trainer Michael Hewitt has pulled off a major upset at Rosehill with outsider Sassaby giving him his first metropolitan win.

Sassaby, the only horse the Muswellbrook-based Hewitt has in work at the moment, is a family project being bred and raced by the trainer's parents.

The four-year-old was a $41 chance in Saturday's Highway Handicap for country-trained horses with Galaxy Warrior the dominant $1.90 favourite.

With Galaxy Warrior boxed in with no clear passage, Sassaby sprinted down the middle of the track.

The favourite finally got a break but it was too late with Sassaby a three-quarter length winner.

"Mum and Dad bred him so that makes it even better," Hewitt said.

"I'm only training the one at the moment. We did have a little something on him at the odds."

Until recently, Hewitt had been training for three years as an owner-trainer on his farm at Denman.

When his other horses are in work he rotates the small team between Muswellbrook racecourse and the farm which he said worked well.