Palpatine didn't underestimated Mace's strength. It wasn't about Mace, he was just a pawn. Anakin was main goal in Palpatine's plans. Palpatine knew, that Anakin doesn't care of his freedom etc. he just wanted him alive. When Mace "defeat" him in sword fight, he was counting that Mace will try to kill him, but Mace wasn't angry enough. So he used FL to bring Mace deeper into vaapad. After that, Mace's judgement was clouded by the dark side, and he decided to do something, what Anakin cannot allow. It forced Anakin's do choose, and it was obvious for Sidious, that Anakin willl choose to defend his life.
So he was toying with Windu all the time. Maybe even Mace was better duelist, but who cares. Palpatine was superior force user, with superior TK, FL, speed... Mace can't block TK with his sword. Even full power lightning would be to much for him.