Had Sai learned, earlier in his life, to question authorities – he probably would have spoken up. He would have shared his doubt with Neji and Tenten, had their Hokage proven to be too eager to finally solve this problem to listen.
But as it was, he had assumed that Kakashi knew what he was doing. There was no way Kakashi would have proposed such a dangerous endeavour, if he wasn't entirely sure that it would work. Right?

"You won't", he had said "That's why I'll deal with him on my on."
To this no one said anything. They just waited by the village's west-gate until Rin joined them and together they started their way towards the cave, Kakashi's ninken trailing behind them, where the sealing circle was already waiting.

Sai, Rin, Neji and Tenten went up the trees, to the hiding-spots from where they would maintain the seal.

Even though Sai would never have admitted it, he was quite curious to see what Kakashi would fight like, if there were neither a team (except for his dogs) nor his Sharingan or any ninjutsu he could rely on.
But the leaves of the tree Sai was perched in grew so thick that he could hardly see anything.

When ever he caught a glance of the two Kakashi's fighting below their feet, it struck him as terribly confusing – he had no idea how the ninken managed to tell those two apart. The only difference he could make out was the lack of killing intend in one of them. (This confused Sai at first, until he remember that naturally, Madara wouldn't want Kakashi to die.)

For the fragment of a second their seal faltered. Sai was already going through plan b in his head when he heard Neji's voice through his earpiece.
"We're still on it", he said. "They entered the cave. Tenten?"
"I'm fine", the girl whispered into her microphone. "Sorry."

They waited for some more time. Each minute made it harder to maintain the seal. Sai could feel beads of sweat collecting on his upper lip and his hands' growing wish to start trembling.

Kakashi really needs to get on with this, he thought to himself. It was easy to sense that he wasn't the only one having trouble maintaining his chakra-flow.

One moment before Sai brought himself to acknowledge that it would be him who would spoil their plan, he could feel the tension leave his body, as the elaborate net of chakra which connected the four of them broke down. Had it worked? Had Kakashi actually managed to defeat Madara? Sai still pondered on how to adequately put this question, when Rin already asked it out loud.

"Did he do it?", she enquired, her voice a tat too agitated for the transmission via radio.
"I'm not sure", Neji replied.
The leaped down from their trees into the clearing but did not dare to enter the cave.
They could hear that there was still a fight going on. Sai looked over at Neji, wanted confirmation that they should proceed with plan b, but found himself startled.

To be a professional never meant that nothing would ever startle you in combat. It simply meant that you had learned the infinite number of things to expect during a fight. This, however, was something Sai hadn't expected. The swift descend, or the wind, had removed Neji's hood, the hood he had been wearing for the past week, and revealed what the young Hyuga had tried to conceal.

His hair had been cut of, his head shaved blank and bald, robbing him of the second most prominent mark of his clan.

"We've got to do something!", Rin's voice cut through Sai's and Tenten's mutual bafflement.
She had landed the nearest to the cave's mouth, and beckoned the others to join her.
Inside, it was hard to see much, it was dark. Sai could make out a lot of blood, covering the floor and the walls. And somewhere in the middle of it, Kakashi, one Kakashi was kneeling, trembling violently as if fighting against his own movement.
Sai could hear Kakashi's voice (which no doubt, did not belong to him at that moment), cursing quietly as his body refused to move any further.
And then, just like that, he collapsed.