The final error.
1. d2d4  1... g8f6  2. c2c4  2... e7e6  3. b1c3  3... f8b4  4. a2a3 The world champion often chooses this line against the Nimzo-Indian in blitz and rapid. 4... b4c3  5. b2c3  5... c7c5  6. e2e3  6... b8c6  7. f1d3  7... d7d6  8. g1e2  8... e6e5 In return Giri builds the Hubner-Portisch  wall. 9. e2g3 A pawn sacrifice that Black sensibly rejects. 9... h7h5  10. h2h4 A novelty. Carlsen understands these positions in the  smallest detail and is often getting this same pawn structure as Black  when facing the Rossolimo Sicilian. 10... d8e7  11. d4d5  11... c6b8  12. e3e4 Now Black has a problem with his kingside as he cannot castle  easily. 12... c8g4 In order to lock the diagonal d1-h5. 13. f2f3  13... g4c8  14. c1g5 Nevertheless, the pin is extremely unpleasant for  Giri. 14... e7c7 He breaks it at once, but this leaves his kingside in  ruins. 15. g5f6  15... g7f6  16. e1g1  16... b8d7  17. f3f4 Black's kingside crumbles. 17... e5f4  18. g3h5  18... d7e5  19. h5f6 Forcing matters. 19... e8d8  20. f1f4  20... e5g6  21. d1f3 Excellent! The world champion is  ready to spend an exchange for the anti-blocker. By doing so he also  secures the excellent position of his knight on f6 (which is definitely  not weaker than any of the black rooks) and, yes, the h-passer remains  alive. 21... c7e7  22. h4h5  22... g6f4  23. f3f4  23... e7e5  24. f4e5 This trades the opponent's best piece  at once. 24... d6e5  25. g2g4 The steed is supported and becomes a super knight. 25... a7a5  26. g4g5  26... a8a6  27. a1b1 Carlsen wants to block the a6 rook from  entering the battle via the b6 square but forgets about something else. 27... a6f6 Finally some breathing room for the black pieces! 28. g5f6  28... h8h5  29. b1b5  29... h5h3  30. d3e2  30... h3c3  31. b5a5  31... c3e3 Going for the wrong pawn. 32. e2h5  32... e3e4  33. a5a8 Giri must have missed this. The white pieces find a way to communicate from the different flanks. 33... d8c7  34. d5d6  34... c7d7  35. h5f7  35... e4g4  36. g1h2  36... g4f4  37. h2g3  37... f4f6  38. f7h5 Wins material. 38... f6f4  39. h5g4  39... d7d6 The best chance. Black is getting the next-to-last white pawn and needs to  make it to the last one in order to reach the theoretical draw R+B versus  R. 40. g4c8  40... f4c4  41. c8b7 It should be a win, but not as easy as some people think. The black passers can become quite dangerous. 41... c4c3  42. g3g4 That is not the right direction. 42... c5c4  43. b7e4  43... c3c1 In time trouble Giri misses an excellent chance. 44. a8a5  44... c1f1  45. g4g3  45... f1f4  46. e4f3  46... f4d4  47. g3f2 