Amarande: <Travis Bickle> Not really so masterful; as of move 41, the advantage was in fact Averbakh's, but for reasons unknown he drifted quickly into a loss. I suspect fatigue of a long game (it would be interesting to know if it was adjourned after move 40 or if the last moves were played in the same session) may have been a factor, as even Korchnoi's 43rd through 45th moves are somewhat weak and should actually have handed Averbakh back the advantage (Rybka 2.3.2a gives 43 Nf3, 44 Rff3, and 45 Rf3 as the best moves, while encouraging Black to play g5 on all moves 42-45).
The decision to trade Queens, however, is suspect, even if Averbakh could have performed far better in the endgame. Black's advantage lies chiefly in attacking chances against the White King, while White's advantages are the c-file and Queen's side majority which both clearly suggest that the middlegame favours Black while the endgame favours White.
Rybka gives Black's best move 41 as ... Qe4, after which Black has apparently a solid advantage of about 1.2-1.4.
Interesting is the line 41 ... Rg5+, given by Shamkovich and Schiller in the book Saving Lost Positions. S. & S. give this as winning for Black but it appears to be a draw:
<41 ... Rg5+ 42 Kh4> Not 42 fxg5 h4+ 43 Kf3 Qe4#, nor 42 Kf3 Qe4#. <42 ... Qxe5! 43 Qf3> Still pressuring Queen and Rook as he needs to recover the material just lost. Neither can be taken yet because of mate (43 fxe5 Rg4# or 43 fxg5 Qe4+ and mate in two more moves at most) <43 ... Qd4! 44 Kxg5> Forced on account of the threat Rg4+, and even if this wasn't the case, anything else would lose as White would be a piece down. <44 ... Nh7+>
A critical juncture. If now 45 Kxg6, as given by S. & S., then 45 ... Qg7+! wins out of hand: 46 Kxh5 Nf6+ 47 Kh4 Qh6+ 48 Kg3 Ne4+ 49 Kg4 Qg6+ 50 Kh4, and now with 50 ... Nd6 Black wins easily, for White must lose at least a Rook to prevent Nf5# (the defence 51 g4 fails to Qh6+ and Qh3#).
However, there is also 45 Kh4, which S. & S. dismiss with the line 45 ... Qf6+ 46 Kg3 Ng5, stating it "cleans up nicely." In fact, Rybka gives this as the best play against 45 Kh4, but does not find any more than a draw for Black after 47 Qe3 (not 47 fxg5? Qxg5+ ) Ne4+ 48 Kf3 e5! (not 48 ... Nxf2? 49 Rc8+! ) 49 Qxa7 Qxf4+ 50 Ke2 Qxf2+ 51 Qxf2 Nxf2 52 Kxf2 Ra1, and the Rook and Pawn ending appears to be drawn (even as far as depth 22, Rybka gives a flat 0.00 evaluation). All most interesting (and notably, according to S. & S., the position in the losing 45 Kxg6? line has been the subject of chess-blindness by even Grandmasters, who consistently thought the position after 50 Kh4 to be a draw, completely missing 50 ... Nd6!).