Stains from fruit, including tomato sauce, disappear when boiling water is poured over them.
Run the fabric, inside out, under cold water to flush out as much of the sauce as possible. Apply liquid laundry detergent to the stained area and let it soak in cold water for 15-30 minutes. Rinse the fabric with cold water. Next, sponge the stained area with white vinegar, applying the vinegar with a clean white cloth. Rinse with cold water, again. Pretreat the stain with a laundry prewash stain remover. Launder in the hottest water the fabric will allow to get out the stain.