Star Hunter by Andre Norton

Vye crawled weakly from the area of a rock outcrop. The sun, reflected from the cliff side, was a lash of fire across his emaciated body. His swollen tongue moved a pebble back and forth in his dry mouth. He stared dimly down the slope to that beckoning platter of water open under the sun, rimmed with the deadly woodland.

What had happened? They had gone to sleep that first night under the ledge of the dried waterfall. And all of the next day was only a haze to him now. They must have moved on, though he could remember nothing, save Hume's odd behavior -- dull-eyed silence while stumbling on as a brainless servio-robot, incoherent speech wherein all the words came fast, running together unintelligibly. And for himself -- patches of blackout.