The match, which is live on BBC Scotland kick-off 7.05pm, sees second-placed United face an Ayr team six points behind in third spot with a game in hand.
If Robbie Neilson's team drop points this evening then leaders Ross County will have the chance to clinch the title at home to Partick Thistle on Saturday.
Ayr have won all three meetings between the teams this season, including a 5-0 victory on their last visit to Tannadice in November when Lawrence Shankland scored four goals.
