Chess games that advanced endgame theory Single games of chess can routinely have a significant impact on opening theory, of course, but it should be quite uncommon for a single game to significantly advance endgame theory, since that's a phase of the game where we can and do have some absolute knowledge that will never change, and since the chance combination of a situation that is ripe for something fundamentally new to be discovered along with a player capable of doing the discovering in the course of a game is a rare thing.

Nevertheless:


  I am looking for pointers to historical instances where the course of a game's play revealed new theoretical endgame knowledge that was unknown before that game was played, e.g. an unknown winning method in a certain type of endgame, or an unexpected drawing resource that had escaped notice until that point.
In a game at the 1954 Olympiad in Amsterdam, Botvinnik had the white pieces against Nikolay Minev of Bulgaria, and reached the following position after 57. Qxe6:

[FEN "8/8/4Q3/k6K/8/6P1/8/q7 w - - 0 1"]
[StartFlipped "0"]


In Half a Century of Chess, Botvinnik writes,


  Ten years prior to this game I had such an ending against G. Ravinsky ... There I did not understand the specific character of the ending and I tried, as in a rook ending, to keep my king on the eighth rank to support the promotion of the pawn at g8 and, this made the win most difficult.


That earlier game against Ravinsky had been analyzed by Keres, but still little had been properly understood about this ending. For instance, in a note after 61. ... Ka4 in the Minev game, Botvinnik points out, "Minev willingly occupies a4 with his king, as this was recommended by Keres in a well-known article where he analysed the ending of my game with Ravinsky." Instead, it turns out that the best place for the black king in this situation would have been the corner a1 square. There was an adjournment at move 73, and Botvinnik notes,


  Although I did not find the correct plan before the adjournment, nevertheless I instinctively avoided moving my king to the eighth rank. After thorough analysis I finally found the right way of playing this ending. ... The winning plan consists in placing the white king on the same rank (or file) as the black king, or on the adjacent one. In this case White has a good chance of sheltering his king from checks. Once the method is found, of course, it looks simple.


Finally, after White's 91st move, Botvinnik's plan had reached fruition:

[FEN "8/6P1/8/2KQ4/k7/8/7q/8 w - - 0 1"]
[StartFlipped "0"]



  Black has as many as three checks, but they all lead to the exchange of queens. Here the game ended, Black resigned, but the first page of genuine theory was opened on the ending "queen and pawn against queen".


For yet another source commenting on the ending to this particular game,
here is Giddins in his The Greatest Ever Chess Endgames:


  Relatively little was known about this type of endgame at the time this game was played, and despite a few inaccuracies in the play, Botvinnik first demonstrated the winning method for such an ending in this game. ... Without the aid of either tablebases or any substantial theoretical practice, apart from his own game against Ravinsky years before, Botvinnik effectively "solved" this ending in his adjournment analysis, identifying all the key elements of the winning method in such positions.