Random nerd thought of the day:

George Lucas has said that Mace Windu won his fight with Sidious,  but I don't think that's entirely true.  Mace did put Sidious on his back, but it was all by Sidious's design and at no point was Sidious in any danger from Mace Windu.  Below are some points that I think support my conclusion.

(1)  Although now resigned to "legends" and not canon I believe until the novel Darth Plagueis until contradicted it is still valuable for understanding Star Wars. In that novel after months of intense Sith meditation Darth Plagueis and Sidious tip the balance of the Force to favor the dark side clouding the Jedi's foresite and strengthening their own.

(2)  Mace walked into the Chancelor's office at a serious disadvantage.  Whereas Sidious had decades to study Mace and understand his fighting style and abilities with countless previous fights during the Clone Wars by Mace to study him Mace didn't have any knowledge of Sidious's fighting styles or abilities.  

(3)  Everything that had transpired had done so according to Sidious's design and everyone who entered the Chancelor's office was essentially there at his invitation. The timing is too perfect and it becomes sillier and sillier to chalk so many things up to coincidence, especially when we're discussing Sidious who is a master manipulator.

(4)  In the novel Darth Plagueis there is a part where Plagueis tells Sidious that a Sith cannot have the full power of Sith lightening until he takes some of it himself.  So, when Mace is redirecting the Sith lightening back into Sidious what he is really doing is charging Sidious up in power like a damn Dark Side battery.   That is why Sidious screams "unlimited power" when he finally unleashes his Force lightening on Mace sending Mace through the window.  Sidious could at long last take the Force lightening and allow himself to be disfigured by it because he could drop his mask.  Notice that after receiving the Force lightening into his face that Sidious although pleading with Anakin to help him because he was "too weak" just stand up on his own after Anakin slices off Mace's arm.  Sidious after Mace goes through the window doesn't act injured in the slightest or even out of breath.  Sidious's voice now resonates with an almost echo as he says "rise" and that is to signify that he has come out of that duel with even greater power than he went into it.  Sidious never has to go into a medical capsule or bacta tank, he has full command of faculties and is able to carry out the Sith grand plan flawlessly immediatley after that fight.  Shortly after that fight Sidious even has the strength to go toe to toe with Yoda himself in the Senate.  

Sidious played Mace like a fiddle to in turn play Anakin.  At no point was Sidious ever in danger from Mace and he had full command of that fight. Even as he's pleading for Anakin to help him you can see Sidious doing a pretty poor job of hiding the joy in his eyes when manipulating Anakin and it's definitely not a look of fear.  I believe in a number of interviews when asked if Sidious won the dual the actor who played him, Ian McDiarmid has stated that he won and I believe he definitely played that scene that way.  What do you guys think?    

Getting thrown out of window kind of settled who won that duel.

OP is correct.

George Lucas saying afterward that Mace Windu
won the lightsaber duel is as much bullshit as
J.K. Rowling retroactively making Dumbledore gay
when nothing implied so in the books or films.