What Was Fischer Thinking?

In my introduction to Fischer - Unzicker, Zurich 1959, I mentioned that there weren't too many differences of opinion between Fischer and Kasparov on the game. I overlooked one buried in the notes.

As Fischer noted, the position in the diagram had already appeared in one of his earlier games: Fischer - Shocron, Mar del Plata 1959, 60 Memorable Games no.6. He has just sacrificed a Pawn and the question is whether the sacrifice is sound. His analysis to the earlier game went

20...Bxg5 puts White's concept to the crucial test. After 21.Nd5! Bxc1 22.Nf6+ Kh8 23.Raxc1 Rf8 24.Qg3 Qc7 25.Qg5 Kg7, White can force a draw with 26.Nh5+, or try for more with either 26.f4 or 26.Re3.

In other words, Fischer thought he could force a draw or play for a win.

Zurich 1959
Unzicker, Wolfgang

Fischer, Robert
(After 20.g4-g5)
[FEN "3rr1k1/4bp1p/p1q1b1p1/npp1p1P1/4P3/2P1NQ2/PPB2PP1/R1B1R1K1 b - - 0 20"]

Kasparov repeated Fischer's analysis and dimissed the tries 26.f4 and 26.Re3 with

After 26...h6! he is unable to achieve any more.

In other words, Fischer's opponents also weren't obliged to play for a win. They could force the draw themselves. With this in mind, the sequence starting 20.g5 is good for no more than a draw. Since the move 26...h6 isn't particularly difficult to find, what was Fischer thinking?

Both Shocron and Unzicker continued 20...Nc4 21.Ng4 Bxg4 22.Qxg4. Shocron continued 22...Nb6 ('!' by both Fischer and Kasparov), while Unzicker played 22...f6 ('?', ditto).