A mistake.
1. c2c4 Just like So did yesterday, Aronian decided to use the English  Opening in the must-win game. 1... c7c5 The symmetrical move is  considered the most solid answer against the English. 2. b1c3  2... g8f6  3. g2g3  3... d7d5  4. c4d5  4... f6d5 Now it is a reversed Accelerated Dragon. 5. f1g2  5... d5c7  6. d2d3  6... e7e5  7. c1e3  7... f8e7  8. a1c1 The extra tempo promises White some extra  possibilities. He can, for instance, attack the pawn on c5 before  castling. 8... c7e6  9. g1h3  9... e8g8  10. e1g1  10... b8c6 Allowing White a chance to ruin his pawn  structure. 11. g2c6 Aronian does not need a second invitation. In the usual Dragon,  this is often a good equalization strategy. In the long run, White hopes  to pick up the weak queenside pawns. But before the pleasant endgame, he  needs to survive the unpleasant middlegame. 11... b7c6  12. f2f3  12... a8b8  13. b2b3  13... f7f5 Aronian's plan is clear, but so is Nepomniachtchi's. He wants  to throw everything against the white king that badly misses the  fianchettoed bishop. 14. h3f2  14... h7h5  15. g1h1 This prophylaxis does seems unnecessary. 15... g7g5  16. c3a4  16... b8b5  17. e3d2  17... g5g4 It starts to heat up around the white king, and  Aronian's next move does not make things better.