I peel and seed fresh tomatoes before using them in a sauce otherwise there is a lot of extra liquid. The best way to lose the liquid is by slowly reducing it on a low flame until it reaches the consistency you want. You need to stir regularly otherwise the ingredients could burn. A quick fix is to add a little flour to the sauce but you run the risk of a floury taste and it adds extra calories.
~ Gloria Kobrin of Kosher Cookbook