1.d4 [A radical change from the previous meeting between the same opponents where White had opened 1.a3 a move that has surprisingly been played several times by Carlsen even against players rated over 2700 Elo.]
1...Nf6 2.c4 e6 After this two move sequence a vast range of different openings could occur such as the Bogo-Indian, Queens-Indian, Nimzo-Indian, Queens Gambit Declined as well as the Benoni, Benko or Blumenfeld gambits amongst others.
3.Nc3 c5 4.Nf3 [4.e3 is also a frequent White choice here and after 4...d5 5.Nf3 a6 6.Bd2?! Black can transpose to a good version of the Queens Gambit Accepted with 6...dxc4 7.Bxc4 b5 ]
4...cxd4 Much more frequently other moves are played here, from the large number of playable options available, but the text was an at the board decision to try and avoid any potential opening preparation by White.
5.Nxd4 Bb4 Here Black has even more playable options from which to choose and again the text is not one that I have played often. Perhaps White was not expecting the selected line as he began to consume large amounts of time over the next few moves.
6.Bd2 [A major alternative is 6.g3 transposing to the Romanishin variation of the Nimzo-Indian as has occurred in a number of my correspondence games.]
6...Qb6!? Finally a rare line not played by Black before which gave White more to think about as, apart from attacking d4, the move has a veiled threat to the pawn on b2. [Previously I had played 6...Nc6 when one game went 7.Nxc6 bxc6 8.a3 Bxc3 9.Bxc3 d5 10.e3 0-0 11.Qd4 Re8 12.Qe5 Ba6 13.Rd1 Qb8 and Black had a good position in Beach-Donnelly, Teesside International (Thornaby) 1973.]
7.e3 This is Ok for Black as are the other moves played here: [7.Nf3 0-0 8.e3 h6 9.a3 Bxc3 10.Bxc3 Qd8 Tong-Nguyen,VIE-ch Bac Giang 2016.; 7.Nb3 0-0 (7...Bxc3 8.Bxc3 Ne4 9.Qd4 Nxc3 10.Qxc3 0-0 Marchand-Weenink,Amsterdam Zevencamp 1920.; 7...Nc6 8.e3 Preake-Timofeev, Polugaevsky Memorial 2012. (8.Qc2 a5 9.e3 a4 10.Nc1 Grams-Assmann, Niedersachsen-ch Wingst 2002.) ) 8.e3 d5 9.a3 dxc4 10.Bxc4 Be7 Pali-Bakos,Ajkanborostyan Cup 2000.; The untried 7.a3 Bxc3 8.Bxc3 Ne4 ; and 7.Nc2 give White a very small edge according to Chessbase Common Analysis.]
7...Nc6 [7...e5 is poor because of 8.Ndb5+/- with idea of (and 8.Nf3 is also promising for White 8...d6 9.a3 Bxc3 10.Bxc3 Ne4 11.Bd3 Bf5 12.c5 dxc5 13.Qa4+ Nd7 14.Bxe4 Bxe4 15.Qxe4 Kirlyonkov-Leshenko,Yuzhny open 2009.) 8...a6 9.Na4+- ; but 7...a6 is playable 8.a3 (or 8.Bd3 0-0 9.0-0 Be7 10.Qc2 d6 11.a3 Nbd7 12.b4 Qc7 Minald-Stupak, Minsk Bronstein Memorial rapid 2015.) 8...Bxc3 9.Bxc3 Ne4 10.Qc2 Van Diijk-Petrou,Box Hill open 2001.; and simply 7...0-0 is also fine for Black.]
8.Nf3 White players have tried several other moves here but not obtained an advantage. These include [8.Nxc6 ; 8.a3 ; 8.Nc2 ; 8.Na4 ; 8.Ndb5 ; and finally 8.Qb3 ]
8...Be7 Retaining the bishop so as to avoid giving White a tempo with a3, and also controlling the central dark squares especially d6 and f6. The move also eyes the vulnerable b2 pawn. That Black has a viable position is shown by the fact that other reasonable ways to treat the position exist such as [8...d5 9.cxd5 Bxc3+/- (9...exd5= ) 10.Bxc3 exd5 11.Bxf6 gxf6 12.b3 Be6 Natal Gomes-Bueyuekaltay, IECG 2006.; 8...0-0 9.Be2 Bxc3+/= (9...d5= ) 10.Bxc3 Ne4 11.Qc2 d5 12.0-0 Aranaz Murillo-Pena Gomez, ESP-ch El Sauzal 2010.]
9.Na4 The bishop retreat seemed to surprise White who spent a long time considering his reply and thus ended up a long way behind on the clock at an early stage in the game. [Whilst the text is OK it doesn't really trouble Black and there is a risk the knight might end up offside on a4. Hence 9.Bd3 is more solid as Black is not yet ready to take on b2 due to the lines 9...Qxb2 10.Nb5 Ne5 11.Nc7+ Kd8 12.Nxe5 Kxc7 13.Nxf7 Bb4 14.Bc2+/- (and 14.Nxh8 Bxd2+ 15.Kf1+/= ) ]
9...Qc7 10.a3 b6 [Deciding to delay castling to set up a potentially strong bishop on b7. In that case, when White castles kings-side, Black's pieces will begin to start aiming at that section of the board. Instead 10...d6 is also fully playable, when for instance, 11.Bd3 0-0 gives Black a solid position.]
11.Rc1 Bb7 12.Bd3 [12.c5 trying to exploit the position of the rook facing the Black queen is dubious due to 12...0-0 13.cxb6 axb6 14.Bd3 Rfc8=/+ ]
12...Rc8 13.0-0 Ne5 Ambitious due to White's now serious time trouble. The more solid [13...0-0 14.Bc3 d6 is very comfortable for Black.]
14.Nxe5 The best move retaining equality. The retreat [14.Be2 gives Black a winning attack as the following lines show 14...Neg4 (14...Nfg4 is much less effective as White is fine after 15.Nxe5 (but not 15.h3 Nxf3+ 16.Bxf3 Qh2# ) 15...Nxe5 16.c5 Qc6 17.f3 b5 18.Nc3 a6 19.b4= ) 15.h3 (15.g3 is much worst 15...h5 16.h4 Ne4 17.Be1 g5-+ ) 15...h5 (15...Bxf3 is also less effective 16.hxg4 Nxg4 17.g3 Bxe2 18.Qxe2 Nf6-/+ ) 16.hxg4 (16.c5 Bxf3 17.hxg4 hxg4 18.Re1 Qh2+ 19.Kf1 Qh1# ) 16...hxg4 17.g3 Ne4 18.Kg2 Rh3-+ as Ng5 follows.]
14...Qxe5 Now Bd6 may be an option for Black attacking h2. Similarly h5 and Ng4 may also be on the cards at some point.
15.f4 OK but leaving some weaknesses. So better was the more solid [15.Qe2 0-0 (attacking options are here premature eg 15...Qg5 16.e4+/= ; or 15...Bd6 16.f4+/= ; and also 15...h5 16.f4+/= ; but 15...Be4 is a secure line leading to even play after 16.Nc3 Bxd3 17.Qxd3 0-0 ) 16.Nc3 Rfd8 with about a level game.]
15...Qd6 16.Qe2 Bxg2!? White had less than about 1 minute to reach the time control on move 30 hence this move was selected as it could lead to enormous complications and is not as simple as it appears at first sight. The calmer [16...Be4 would lead to exchanges and about an even game via 17.Bxe4 Nxe4 18.Rfd1 Qc6 19.Nc3 0-0 20.Nxe4 Qxe4= ]
17.Qxg2? White collapses as this loses outright but White had a mere 9 seconds left on his clock at this point in the game. [Instead 17.Kxg2 leads to severe complications 17...Qc6+ 18.e4 (not 18.Qf3 Qxa4 19.e4 Qc6 20.e5 Qxf3+ 21.Rxf3 Nh5=/+ as Black is a safe pawn up.) 18...Qxa4 19.e5 Qc6+ 20.Kh3 leads to very obscure play especially if White invests another pawn to retard Black's development 20...Ng8 21.Be4 Qc7 22.f5 exf5 (22...g5 is bad 23.f6 Bf8 24.Bc2 h6 25.Ba4+- ) 23.Bd5 g6 24.Bf4 Qd8~~ and White has significant play for the two pawns sacrificed.; 17.Nxb6 is another tricky line to evaluate in time trouble 17...Qxb6 (Black also has the unclear line 17...Bxf1 18.Nxc8 Qxd3 19.Kxf1 Qe4 ) 18.Kxg2 (18.Qxg2 Qxb2-/+ ) 18...Qxb2-/+ ; Whilst 17.Bb4 also had to be considered but loses in short order after 17...Bxf1 18.Bxd6 Bxe2 19.Bxe2 Bxd6-+ ]
17...Qxd3 18.Rc3 [Neither 18.Rfd1 Rxc4-+ ; not 18.Qxg7 Rg8-+ help White at all.]
18...Qe4 Here White lost on time but the game was beyond hope. After [18...Qe4 19.Qxe4 Nxe4 20.Rc2 Nxd2 21.Rxd2 Rxc4-+ White is two pawns down without any compensation. A game dominated by White's handling of the clock.] 0-1