The weather in Sydney is set to determine where Sweet Thomas will progress his Sydney Cup campaign.

The Matthew Smith-trained stayer is entered for Saturday's Mornington Cup (2400m) in Victoria along with the N E Manion Cup over the same distance at Rosehill.

Smith said he would accept in both races before deciding on where the gelding will start.

He says Sweet Thomas needs to run this weekend if he is to keep on track for the Sydney Cup (3200m) at Randwick on April 11.

"It all depends on the track and if the track is OK at Rosehill then I'll probably stay and run him up here," Smith said.

"But if the track looks like being no good then I'll more than likely send him down.

"I can't afford to miss a run with him and they're the only two options for him.

"I could have taken him to the Albury Cup but I've already got one (Attorney) in there, so it will be one of these two races."

Sweet Thomas has impressed Smith with his two runs this campaign, although the gelding has finished out of the placings on each occasion.

First-up over 1900m in the Parramatta Cup at Rosehill last month, Sweet Thomas made ground to finish fourth behind Mustajeer before running seventh over 2000m in the Randwick City Stakes won by Shared Ambition.

"He's looking for the 2400 metres now and I reckon his two runs this time in have been excellent," Smith said.

"They've been slowly run races and he ran home well in both of them.

"It shows me the horse is going in the right direction."

Smith said Sweet Thomas could head to the Chairman's Quality (2600m) at Randwick on April 4 ahead of the Sydney Cup, but the gelding's performance in either the Melbourne or Sydney race will determine if he needs another start.