Grischuk - Malakhov
Grischuk is an 1.e4 player, but has started playing 1.d4 in some of his games a few years ago.
Against 1.e4 Malakhov plays the Gurgenidze variation of the accelerated dragon as a drawing weapon (1.e4 c5 2.Nf3 Nc6 3.d4 cxd4 4.Nxd4 g6 5.c4 Nf6 6.Nc3 d6 7.Be2 Nxd4). And a drawing weapon this line is in the hands of Malakhov - other results are very rare if he gets to play it. Grischuk would need to have some excellent preparation to beat Malakhov in this line.
Grischuk has been playing 3.Bb5 against the 2..Nc6 Sicilian's for some time now, and this may well be what we will see in the match.
Or Grischuk may prefer 1.d4, which Malakhov usually counters with the Slav with an early ..a6.
Malakhov - Grischuk
With White Malakhov is not nearly as predictable in his openings as with Black. He plays all the major opening moves 1.e4, 1.d4, 1.Nf3, and 1.c4 regularly.
Against 1.e4 Grischuk plays the Marshall, which has completely replaced any other closed Spanish lines he played earlier. He recently made the Najdorf another main part of his Black repertoire. He previously called the Najdorf losing by force after some of his nice wins with the English attack against it, but apparently changed his opinion.
Malakhov rarely plays into these main lines, though. Against 1..e5 he has played the Scottish four knights game (1.e4 e5 2.Nf3 Nc6 3.Nc3 Nf6 4.d4) and recently 3.Bc4, where we could expect the game to continue with 3..Bc5 4.c3 Nf6 5.d3.
Against the Najdorf, Malakhov has tried almost every system known to man, including sidelines such as 2.Na3.
Against 1.d4 Grischuk has played the Slav or the Nimzo/Queen's Indian recently.
A tough opponent for Grischuk. Malakhov's super-solid play with Black should be useful in a match, while he is unpredictable with White. If Grischuk manages to take the lead in the match though, it will be interesting to see if Malakhov avoids the Gurgenidze and plays something sharper.