My own thoughts on the matter.
If Anakin had not being coming to his rescue Sidious would have never stopped the Lightning barge are just kept on until Windu was overwhelmed, then fried him.
Windu may have legitimately won the Lightsaber portion of the fight, but Windu was on losing end of that Lightning vs Saberblock exchange, as few more seconds and toast. But then Anakin came in so Sidious went for he double win, dead Windu and corrupted Anakin.
It would be nice to think Windu actually did stand a chance, unfortunately, the plot armor was strong with Sidious.