Either way, Sidious won the fight. So Mace never actually "beat" him. It is more correct to say that Mace gained the advantage and then lost it.
It has always been my opinion that Sidious did throw the fight. It's his signature move. He did it against Galen Marek, for instance. He let Starkiller pound him and then right when Starkiller put his lightsaber down, he popped up like a jack-in-the-box and killed him easily.
He tried the same trick on Luke, but it didn't work.
If there is an official canon source that explicitly states that Palpatine in fact was defeated fair and square by Windu, please give me a link.