I reread the passage in the novel "The Revenge of the Sith". It's really hard to tell whether Sidious lost on purpose as you suggested or is genuinely defeated by Mace Windu.
I will try to elaborate on this a little and you can choose whether you agree with my interpretations or not. For this I will first quote some important passages from the book.
Setting the stage: The chronology of the battle with text passages
Firstly, it is explicitly said that the lightsaber fight reached an impasse. I'd say that Mace and Sidious were equal when it came to lightsaber battle which was mainly due to the Vaapad fighting style and Mace's great skill with the lightsaber in general. This went on until Anakin arrived.
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. [...] But there was still only the cycle of power, the endless loop,
no wound taken on either side, not even the possibility of fatigue.
Which might have gone on forever, if Vaapad were Mace's only gift. The fighting was
effortless for him now; he let his body handle it without the intervention of his mind.
While his blade spun [...] his mind slid along the circuit of dark power, tracing it back
to its limitless source. Feeling for its shatterpoint. He found a knot of fault lines in
the shadow's future; he chose the largest fracture and followed it back to the here and the
And it led him, astonishingly, to a man standing frozen in the slashed-open doorway. [...]
The chosen one was here.
I guess, Mace thinks Anakin (being Sidous' shatterpoint) will help him ending the battle. Mace and Sidious both feel the end of the battle approaching. Then Mace is able to disarm Sidious taking advantage of the fear being radiated by Sidious.
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 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. Out where
Mace could flick his blade in one precise arc and slash the shadow's lightsaber in half.
Mace puts his victory down to Sidious' own fear.
"You lost for the same reason the Sith always lose: defeated by your
But then Sidious counter attacks.
"Fool!" His voice was a shout of thunder. "Do you think the fear you
feel is mine?"
Lighting blasted the clouds above, and lightning blasted from Palpatine's hands, and Mace
didn't have time to comprehend what Palpatine was talking about; he had time only to slip
back into Vaapad and angle his blade to catch the forking arcs of pure, dazzling hatred
that clawed toward him. Because Vaapad is more than a fighting style. It is a state of mind:
a channel for darkness. Power passed into him and out again without touching him.
And the circuit completed itself: the lightning reflected back to its source.
Mace was able to hold the lightning at bay with his lightsaber and wants Anakin to help him. But Sidious didn't seem to be frightened of Anakin.
The key to final victory. Palpatine's shatterpoint. The absolute
shatter-point of the Sith.
The shatterpoint of the dark side itself. Mace thought, blankly astonished, Palpatine trusts
Sidious brought Mace Windu on the brink of defeat with his force lightning but then stopped it, apparently because of exhaustion.
Palpatine still made no move to defend himself from Skywalker; instead
he ramped up the
lightning bursting from his hands, bending the fountain of Mace's blade back toward the
Korun Master's face. [...] This was beyond Vaapad; he had no strength left to fight against
his own blade. [...]
Mace's blade bent so close to his face that he was choking on ozone. "Anakin, he's
too strong for me—"
"Ahhh—" Palpatine's roar above above the endless blast of lightning became a fading moan of
despair. The lightning swallowed itself, leaving only the night and the rain, and
an old man crumpled to his knees on a slippery ledge. "I... can't. I give up. I... I am
too weak, in the end. Too old, and too weak. Don't kill me, Master Jedi. Please. I
Then Sidious gives up and surrenders, Mace wants to kill him but the Sith Lord's manipulations of Anakin come to fruition and Anakin cuts off Mace's sword-hand.
"I need him alive!" Skywalker shouted. "I need him to save Padme!"
Mace thought blankly,
Why? And moved his lightsaber toward the fallen Chancellor. Before he could follow through
on his stroke, a sudden arc of blue plasma sheared through his wrist and his hand tumbled
away with his lightsaber still in it and Palpatine roared back to his feet and lightning
speared from the Sith Lord's hands and without his blade to catch it, the power of
Palpatine's hate struck him full-on. He had been so intent on Palpatine's shatterpoint
that he'd never thought to look for Anakin's.
It's been a little bit wordy introduction but I wanted to set the stage properly for my answer.
Since I have read the novel "Darth Plagueis" by James Luceno, where Darth Sidious is described as a really powerful, deceitful and foresightful being, I am willing to give him the credit of losing on purpose against Mace Windu only to be saved by Anakin and thus dragging him fully over to the Dark Side.
In my opinion Sidious knew that he couldn't win against Mace Windu in a lightsaber duel (passage 1).
So he would have thought of a different way to win the battle either making use of his stronger force talents (e.g. lightning) or trickery.
He used Anakin's arrival to fool Mace into thinking he's feared of losing and weakened by his fear and let Mace disarm him (passages 3 and 4).
Maybe Sidious drew on Anakin's fear to get the right appearance.
Mace felt that Anakin is Sidious's shatter-point (passage 5) which might have further weakened any suspicions he might have had about this sudden victory over the Sith Lord. (Maybe this is foreboding of Darth Vader killing the Emperor in "The Return of the Jedi".)
After being disarmed Sidious nearly caught Mace off guard with his force lightning. I'm not sure whether he thought he could kill Mace this way, since a normal Jedi wouldn't have been able to block the lightning only Mace's vaapad technique enabled him to withstand it, or whether he counted on Anakin from the beginning. The last interpretation is supported by passage 5 and by the fact that Sidious brought Mace Windu on the brink of defeat with his force lightning but then stopped it (passage 6) although he had some power left (passage 7) to finish him off after Anakin had intervened. (But maybe he really was exhausted, it's difficult to tell.)
I hope my statement was helpful and convincing. There are of course other interpretations possible.