What I see is, Sidious is more focused in force skills to start with, so it is not impossable if he would lose to a saber master, especially one that lingers close to the edge of the darkside on top of his light side skills. Its a matter of combat style, really. The fight starts well within saber range... so Mace got the upper hand there, even if its only a bit.
And, supposely Sidious IS beaten and killed by Mace, its still a win for the darkside. Mace killing Sidious right in front of Anakin would basically shift Mace to the darkside, eventually if not immediately.