Leading Melbourne apprentice Beau Mertens has reached another milestone in his burgeoning career, bringing up his 50th metropolitan winner at Sandown.

Mertens' win on the Robbie Griffiths-trained Single Note was his 40th city win this season and as his 50th overall, his claim will be reduced form 2kg to 1.5kg.

With acceptances already taken when he brought up No.50, Mertens can ride with the 2kg claim at Moonee Valley on Friday night and the metropolitan class meeting at Mornington on Saturday.

Single Note was the second leg of a double for Mertens who opened the meeting with victory on the Mick Kent-trained Tenappy Ladies.

The son of retired jockey Peter Mertens, the 19-year-old said it was particularly satisfying to win on Single Note for Griffiths.

"I'm happy to get that win. I rode my first city winner for him," Mertens said.

"I've been putting in the work and the trainers have been rewarding me."

Mertens' 40 winners put him second on the Melbourne premiership behind titleholder Craig Williams.

He is two in front of Dwayne Dunn who also posted a Sandown double including victory on the Chris Waller-trained English import Richard Of Yorke who scored his first win since beginning his Australian career early in 2016.

Apprentice Ethan Brown had a mixed day at Sandown with his success aboard the Doug Harrison-trained Alrouz soured by a careless riding suspension.

But nothing could dampen Harrison's spirits when nine-year-old Alrouz crossed the line for his first win since May last year as a $21 chance.

"He's a great old horse," Harrison said.

"It seems that long since we've had a winner although we haven't had many runners.

"But it's been pretty tough."