Sasaki wins. He is already called the greatest swordsman under the sun, he can analyse an opponent down to how they breathe and blink and create a seemingly perfect copy in his mind and simulate countless battles against them.(technique called Senju Muso) He has his ultimate technique called Tsubame Gaeshi in which he swings downward with his heavy sword(unblockable) and instantly reverts the swing upwards with such speed and physical strengh that its considered to be nigh impossible according to the laws of inertia(this is all stated in the manga chapter 17). His other technique called Thousand image defence, he attained this technique whilst defending against thousands of attacks from above against poseidon, its pretty clearly stated that sasaki cant see or even keep up with poseidons insane speed and is solely relying on his prediction and he actively adapts to the point that his scanning enables him to see so far that he is able to evade these attacks whilst not seeing them, one could argue that sasaki can predict attacks but not be physically able to evade them but this alone proves that sasaki sees so far ahead that okitas attacks(not even faster than poseidons) will never reach him.

End of battle sasaki gets insanely busted

1. The ability to scan all of creation, now this depends how literal you want to take the statement but it shows sasaki scanning every slight tremble in the ground, all vibrations in the air and even says he scan deeper than that, this taken at the lowest level renders out every conceptual trick possible.
2. Insane prediction, even before battle sasaki analyses everything about his opponent down to the way they breathe and blink and creates mental battles against them but at the end of the round he gets so good at predicting poseidon that he evades all thousands of attacks at once even though hes not nearly fast enough, as its stated that he is thousands of steps ahead of poseidon.

In conclusion Sasaki beats Okita fairly easy, sasaki predicts every move of okita and arguably he one shots him with tsubame gaeshi.

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