Tarjan stated in his analysis of this game that at the time of adjournment, Portisch apparently concluded that even though he was likely to end up a pawn up in an endgame where he'd have two pawns to Spassky's one pawn, it would nevertheless be impossible to win, especially since he was "facing, in effect, two former world champions"---Spassky and his second, Smyslov.

On top of that, Smyslov was well-known to be one of chess history's greatest endgame players (no argument there). Thus, once Portisch convinced himself in the tranquility of his hotel room that the ending was drawn, what reason would he have to doubt that the Spassky-Smyslov duo would find the proper drawing method ?

But then Tarjan made a rather intriguing point, in my view. He gave some extremely detailed analysis of this endgame to show why Spassky could have held the draw despite being a pawn down, and he actually went on to say....

...."if the reader can understand the ins and outs of the following analysis to the point where he could have, say, drawn the position against Portisch, then he can be assured that he has the potential to reach USCF master strength."

Tarjan stated that the ability to "dissect" these kinds of endgames is actually an extremely effective test of one's chess ability, thus making the aforementioned point.