The duel between Sidious and Windu is not a very good measurement of combat skill. It says all over the place that a large part of Sidious' style was to draw the enemy in and let them think they had the upper hand. He had nothing to gain by killing Windu before Skywalker arrived on the scene.
All evidence points to Sidious drawing out their duel before allowing himself to be disarmed, knowing that he could still kill Windu with lightning if he had to, but would rather allow Skywalker to arrive and manipulate him into completing his fall to the Dark Side. So we don't know if Windu was actually skilled enough to defeat Sidious.
They actually fought at a stalemate.
Vaapad made him an open channel, half of a superconducting loop completed by the shadow; they became a standing wave of battle that expanded into every cubic centimeter of the Chancellor's office. There was no scrap of carpet nor shred of chair that might not at any second disintegrate in flares of red or purple; lampstands became brief shields, sliced into segments that whirled through the air; couches became terrain to be climbed for advantage or overleapt in retreat. But there was still only the cycle of power, the endless loop, no wound taken on either side, not even the possibility of fatigue.
--Taken from Revenge of the Sith
Windu only won after exploiting a fear from Sidious(when really it was coming from Anakin.)
He could feel the end of this battle approaching, and so could the blur of Sith he faced; in the Force, the shadow had become a pulsar of fear. Easily, almost effortlessly, he turned the shadow's fear into a weapon: he angled the battle to bring them both out onto the window ledge. Out in the wind. Out with the lightning. Out on a rain-slicked ledge above a half-kilometer drop. Out where the shadow's fear made it hesitate. Out where the shadow's fear turned some of its Force-powered speed into a Force-powered grip on the slippery permacrete.
"For all your power, you are no Jedi. All you are, my lord," Mace said evenly, staring past his blade, "is under arrest."
"Do you see, Anakin? Do you?" Palpatine's voice once again had the broken cadence of a frightened old man's. "Didn't I warn you of the Jedi and their treason?"
"Save your twisted words, my lord. There are no politicians here. The Sith will never regain control of the Republic. It's over. You've lost." Mace leveled his blade. "You lost for the same reason the Sith always lose: defeated by your own fear."
Palpatine lifted his head. His eyes smoked with hate. "Fool," he said. He lifted his arms, his robes of office spreading wide into raptor's wings, his hands hooking into talons.
"Fool!" His voice was a shout of thunder. "Do you think the fear you feel is mine?"
Still though Windu did win against Sidious in saber combat, it wasn't until Anakin's fear came into the situation.