I think that Sidious underestimated Mace as a lightsaber combatant, and I'll accept that he genuinely got bested in that regard. However, he definitely made himself look much weaker than he was just before Anakin showed up. Even without a lightsaber, the Sith Lord wouldn't scramble along the floor helplessly then beg for mercy.
Plus, he clearly was holding back with the lightning a bit. IIRC, I think I recall Palpatine mentioning something about UNNLLIMMETTED POOOWWWEEEERRRRRR!!!!!
So as far as the saber duel goes, Mace actually gained the upper hand, but as a whole fight, Mace wouldn't have won. And Palpatine very skillfully manuevered the entire situation to look perfet for Anakin.
Just think of what would have happened if Sidious and Mace's positions ahd been reversed, with with Sidious standing over an unarmed Mace Windu, threatening him with a glowing red saber at his neck. How would that have effeted Anakin I wonder?
God is a placebo with nasty side effects.