Simply defending the pawn on g5.
1. e2e4  1... e7e5  2. g1f3  2... b8c6  3. f1c4  3... f8c5  4. c2c3  4... g8f6  5. d2d3  5... d7d6  6. c1g5  6... h7h6  7. g5h4  7... d8e7 A rare move that certainly caught Nepomniachtchi by surprise, since in just two moves his position becomes much inferior. The plans with a quick g7-g5 are more common. 8. b2b4 The beginning of a wrong plan. 8... c5b6  9. a2a4 Normally White continues like this after playing b2-b4, but in this position it leads to problems. 9... a7a5  10. b4b5  10... c6d8 The knight is ready to go to e6 and then f4 when Black can then play g7-g5 strengthening its position. Still, it's surprising that the computer already considers Black's position to be much better. Because Black can still castle on the queenside, things are worse for White, since with the inclusion of a5-b5, this flank is closed and Black has a free pass for an attack on the kingside. 11. b1d2  11... d8e6  12. e1g1  12... e6f4  13. d3d4  13... c8g4  14. d1b3  14... e8c8 Black's king is very safe on this side of the board. 15. f1e1  15... g7g5  16. h4g3  16... g4h5 A useful move to prepare g5-g4 or get the bishop to g6. 17. c4f1  17... c8b8 This move is always useful. 18. a1d1  18... h8e8  19. d2c4 A compromising move, since now Black can capture on f3 and weaken the pawn structure. The problem for White is that there's no active plan. If the knight on f4 is captured, then the g-file is opened, and the attack becomes very strong. 19... h5f3  20. g2f3  20... e7e6 Pinning the knight and also controlling the diagonal so that White cannot play Bf3 after capturing the f4-knight. 21. b3c2 Removing the pin. 21... h6h5 This is already a decisive plan. 22. c4e3 The knight heads to f5. 22... e5d4 The clearest path to victory. 23. e3f5  23... d4c3  24. c2c3  24... f6d7  25. h2h4  25... e6e5  26. c3d2 