1.d4 d5 2.c4 c6 The Slav defence to the Queen's Gambit, which is currently all the rage at super-GM level. One small positional detail is that Black's light-squared bishop on c8 often has a bit more scope than is the case in the Queen's Gambit Declined after 2...e6.
3.Nf3 Nf6 4.Nc3 dxc4 5.a4 At this highly sophisticated level, Black doesn't really expect to retain his extra pawn, but White has to take precautions with 5 a4 otherwise Black might well do so.
5...Bf5 6.Nh4 White doesn't want the bishop to have time to settle on the b1-h7 diagonal with h7-h6 (allowing Bf5-h7), so he drives it back at the first opportunity.
6...Bc8 Is Hikaru mimicking Magnus? You may remember Carlsen retreating his bishops to their original squares against Kramnik with powerful effect. But the answer to the question posed is probably "no" - this is a standard retreat here. Black figures that, since White has wasted a move putting his knight on the edge of the board, he may as well put the bishop back on c8 and relocate it somewhere more useful after he has had the chance to kick the knight away from h4.
7.e3 e5 8.Bxc4 [8.dxe5 is a bad idea: 8...Qxd1+ 9.Nxd1 (9.Kxd1 Ng4 10.Ke1 Nxe5 leaves Black a genuine pawn ahead) 9...Bb4+ 10.Bd2 Bxd2+ 11.Kxd2 Ne4+ and Black can follow up with Be6 and claim a slight advantage.]
8...exd4 9.exd4 Be7 10.0-0 0-0 11.Re1 Nd5 12.Nf3 Be6!? Quite a standard looking developing move, but you can bet your bottom dollar (or Norwegian krone) that both these young fellows would have been delving deep into the variations that follow White's next move.
13.Qb3 Now White is piling pressure along the a2-g8 diagonal as well as threatening to take the b7 pawn.
13...Na6 [Black has little option other than to play this. If 13...b6 14.a5! is strong. If Black continues unwarily, e.g. 14...b5? 15.Bxd5 Bxd5? 16.Nxd5 Black cannot capture on d5 with the queen because then the e7 bishop would be lost.]
14.Bd2 [Most experienced players would think hard before daring to play 14.Qxb7 as b-pawns are often laced with poison, but leading diagnostician Dr Fritz seems to think that eating this one would at worst only bring about a slight case of indigestion and, at best, might even be quite nutritious: 14...Nab4 - it's never nice seeing the door slam shut behind your queen, but let's look further - 15.Bxd5 cxd5 16.Bg5!? and White's queen is not in any danger. Black may have some compensation for the pawn in the shape of the two bishops. I imagine Carlsen rejected this line because he wanted something more tangible from the opening.]
14...Nab4 [14...Nac7 15.a5 Rb8 , draw agreed, was Arkell-Gormally, 4NCL 1999, but such an eventuality was not an option for the players of the current game.]
15.Ne4 Bf5 16.Ne5 a5 17.Nc5!? [17.Rac1 is perhaps the more solid option but the text is very challenging and might have led to a very good position for White.]
17...Bxc5 [I suppose a second retreat to the original square with 17...Bc8 was not entirely out of the question here, but then White would continue to build up pressure with 18 Re2, etc.; 17...Nc2 gets horribly complex but after 18.Nxb7 Qc7 (there may be slightly better alternatives) 19.Bxa5! Rxa5 20.Nxa5 Nxe1 21.Naxc6 White emerges with a winning advantage.]