Personally I think Sidious orchestrated the whole thing. My guess is that he saw a picture of the future and figured there was no way he could die against Windu. However I do not think that Sidious saw the whole picture and there are several aspects of the scene that lead me to believe that.
From everything that Lucas has portrayed in regards to Windu, I got the simple sense that he was the best lightsaber duelist that the movies have to offer. Every opponent he faces he's taken down with extreme ease. Sidious was certainly stronger in the force and probably more knowledgable, but that doesn't mean he was the best with sabers. I certainly don't think Maul was more powerful than Qui-Gon, but he certainly seemed to be more skilled with the saber and the distinction proved to be deadly.
I think what happened was that Sidious expected he could hold off Windu until Anakin arrived. Windu suprised him with his superiour saber technique. He then ended up trying to hold Windu off with the sheer power of the dark side. Why else would he disfigure himself? I actually think he was afraid that his visions might have been wrong.
When Anakin intervenes Sidious immediately does a 180 in attitude. My guess is that he saw Anakin as his apprentice in his visions, and when he saw Anakin attack Windu he KNEW his visions were right and he gained his courage back and pressed the attack.
Simply stated I think Windu was the better of the two in regards of lightsabers and got the upper hand. Disagree if you will, but that's my opinion.