Aronian put up an inspired defense to reach a drawn 
endgame K+4p vs B+2p
But just when the draw was in easy reach he committed a terrible blunder:


Here white has an easy draw:
1.h6!   g6
If 1...gxh6 2.gxh6 White plays with a draw in hand.
1.....Kxg5 2.hxg7 and again white has allready a draw.
2.Kb5   Kxg5
3.a4     Kxh6
4.a5     g5
5.c4     g4


The best black can do is reach an endgame Q+p vs Q+B which is a dead draw, since it is white who starts checking.

Instead Aronian played 
1.g6??

1.......,Kg5
2.Kb5, Kxh5
3.a4,   Kxg6
4.a5,   Kf6
This is the essential difference. Because black has first captured on h5 and only then on g6 his king is two lines nearer to the queenside.

5.c4,
Aronian tried 5.a6, Bd5 which does not make a great difference
5......,  Ke5!

And now
6.a6,Kd4! 7.c5,Bc4+ 8.Kb6,Bxa6
or
6.c5,Bd5 and 6.c6 fails on 6...Kd6 -+