Flemington Finals Day proved pivotal in the Melbourne jockeys' premiership race a year ago and it could again have had a big bearing.

Beau Mertens rode a winning treble at Flemington on Saturday, which featured a host of winter series finals, to extend his lead to four over Craig Williams who had to sit out the second city meeting of his careless riding ban.

Williams has six meetings remaining to try to erase the deficit as he chases a sixth premiership.

Williams won last year's premiership, with his five winners on Flemington Finals Day to Dwayne Dunn's one putting him back in the hunt after he started the meeting five behind Dunn.

He went on to claim the title by four.

Mertens has taken a big step towards becoming the first apprentice since Nick Ryan in 2004/05 to win the premiership but knows Williams is still more than capable of bridging the gap.

And while he was rapt to ride the treble at Flemington, he said the title race was not over with Williams sure to have plenty of ammunition with good books of rides at the remaining meetings.

"Four ahead in the premiership, I wouldn't be cheering yet," Mertens said.

"Knowing Craig, he's got a lot of bullets lined up coming up for the next couple of weeks.

"If I keep getting these sorts of rides I should be just kicking along nicely."

Saturday's meeting was a significant one for Mertens who outrode his claim.

He has now ridden 81 city winners in less than 15 months, with 71 of those this season.

"Obviously I'm rapt to get the claim over and done with, especially so soon," he said.

"It's a big achievement for me and something every apprentice wants to do."

Trainer David Hayes has been using both Williams and Mertens regularly and will continue to do so.

"We use them both a lot, and we genuinely are barracking for them both," he said.

"It's up to them.

"Beau helps with a lot of work and Craig is always there and I think he's probably my most Group One-winning jockey. So we go back a long way.

"So I'm happy to support them both."