The thin line between winning and losing at the top level? At lower level, we often blunder many times and do not make the best moves, so there can be multiple ways to win.  But at the top level, for example, during the candidate matches going on now, the masters do not blunder often and they do not make very many mistakes, so I have noticed a theme which is not just in these games, but in others, and that is that the difference between winning and losing can come down to a single pawn for them and they will often fight to the death over one square.  I have noticed that for example if white attacks e4, black will immediately strive to defend it and they will battle over one square.  Is my assessment correct on this?  Are there other factors at the top level?
Note: I'm an average player, who learnt chess about 2 years ago, maybe I'm not the right guy to answer such question but I will try.

I wanted to ask a similar question, maybe I will today or tomorrow. As for your question.

Top level players do make mistakes, maybe they underestimate the opponent, maybe under time pressure, maybe they just don't see the moves, or maybe they are focusing on the attack.

To prove my point I'll show you 2 games and I"ll talk about the mistakes, you can click on the links to watch the videos

Levon Aronian vs Boris Gelfand Round 2 Candidates Tournament 2013

[FEN ""]
[White "Levon Aronian"]
[Black "Boris Gelfand"]
[Event "2013 Candidates Tournament"]
1. Nf3 c5 2. c4 Nc6 3. Nc3 g6 4. e3 Nf6 5. d4 cxd4 6. exd4 d5 7. cxd5 Nxd5 8. Qb3 Nxc3 9. Bc4 Nd5 10. Bxd5 e6 11. Bxc6 bxc6 12. O-O Be7 13. Be3 Qd5 14. Rfc1 Qxb3 15. axb3 Bb7 16. Ne5 O-O 17. Ra4 Rfd8 18. Nc4 Bf6 19. Na5 Rd7 20. Rb4 Ba6 21. Nxc6 Rb7 22. h3 Kg7 23. Rxb7 Bxb7 24. Ne5 Bd8 25. b4 Rc8 26. Bh6 Kg8 27. Rxc8 Bxc8 28. Nc6 Bf6 29. b5 Bd7 30. g4 g5 31. h4 gxh4 32. g5 Bxc6 33. bxc6 Bd8 34. Kg2 Bc7 35. Kh3


If you want a full analysis on that game you can watch the video, I"ll talk about 2 moves that changed the game.

16. Ne5 getting the knight involved, the more brilliant move is 18. Nc4 and 19. Na5 I didn't see that coming! putting pressure on the pawn, basically that knight destroyed black in my opinion. 

Did Boris see it coming? Maybe he did, maybe he just ignored the knight, we all do ignore our opponent's moves, in the end of the middle game, especially if the opponent is attacking, we have to counter-attack, and we don't defend anymore, keep in mind that in this particular tournament, you get 2 hours for the first 40 moves, no time pressure, the game ended in 35 moves.

The next move is 26. Bh6 which finished Boris off. If the king takes, then eventually the knight will fork the black bishop. Didn't he see it? He should see that. The first thing I look at is the major minor pieces of my opponent. If black focused a bit he could've seen it, (Boris is a GM) so white has a knight, a rook and a bishop, it's obvious, has no where to go, if he didn't check the king, so Boris should have thought about what happens if the bishop attacks his king.

Bobby Fischer vs James Sherwin 1957 US Chess Championship

[FEN ""]
[White "Bobby Fischer"]
[Black "James Sherwin"]
[Event "1957 US Chess Championship"]
1. e4 c5 2. Nf3 d6 3. d4 cxd4 4. Nxd4 Nf6 5. Nc3 a6 6. Bc4 e6 7. O-O b5 8. Bb3 b4 9. Nb1 Bd7 10. Be3 Nc6 11. f3 Be7 12. c3 bxc3 13. Nxc6 Bxc6 14. Nxc3 O-O 15. Rc1 Qb8 16. Nd5 exd5 17. Rxc6 dxe4 18. fxe4 Qb5 19. Rb6 Qe5 20. Bd4 Qg5 21. Qf3 Nd7 22. Rb7 Ne5 23. Qe2 Bf6 24. Kh1 a5 25. Bd5 Rac8 26. Bc3 a4 27. Ra7 Ng4 28. Rxa4 Bxc3 29. bxc3 Rxc3 30. Rxf7 Rc1 31. Qf1 h5 32. Qxc1 Qh4 33. Rxf8 Kh7 34. h3 Qg3 35. hxg4 h4 36. Be6 


Let's focus on 30. Rxf7 Rc1 31. Qf1. Black didn't see Rxf7 and of course he didn't see Qf1. That's how you outsmart your opponent! I had to show a game played by Fischer. It answers your question perfectly. Let's say if

Rxf1+Rxf1+
Qxd5Rxf8+
Kxf8exd5

And white has a passed pawn. 

All I want to say is that GMs do make mistakes, they do underestimate the opponent, ignore a move, or maybe they just don't see the game so well. But sometimes they are just outsmarted!