Rublevsky - Grischuk
Grischuk could play the Najdorf, because Rublevsky's 6.Bc4 looked a bit harmless.
But was it? In the fourth game against Ponomariov, after
1.e4 c5 2.Nf3 d6 3.d4 cxd4 4.Nxd4 Nf6 5.Nc3 a6 5.Bc4 e6 6.Bb3 b5 8.O-O Be7 9.Qf3 Qb6 10.Be3 Qb7 11.Qg3 b4 12.Na4 Nbd7 13.f3 O-O 14.Rac1 Rb8 15.c3 bxc3 16.Rxc3 Ne5 17.Rfc1 Bd7
the following position was reached:
I had once the same position with White against Floyd Halwick (email, 1998), but my rook was on d1, not on c1. Rublevsky played 18.Qe1, but much more interesting was 18.Nb6. After 18..Qxb6 19.Nxe6 Qxe3+ 20.Rxe3 fxe6 an interesting material imbalance could have been created. That is what happened in my correspondence game, which ended in a draw. However with the rook on c1 it looks a bit stronger.
On the other hand you can't force this position either, as Black has so many alternatives, not the least of which would be 9..Qc7.
Grischuk can also decide to play 1..e5 and try Rublevsky's Scottish opening.
Grischuk - Rublevsky
I hope Grischuk has read my recommendation to Ponomariov, and will use Khalifman's line against Rubelvsky's Paulsen Sicilian.
After 1.e4 c5 2.Nf3 e6 3.d4 cxd4 4.Nxd4 Nc6 5.Nc3 a6 Grischuk also has played the rare move 6.f3, also once against Rublevsky. This move is so rare that it isn't even mentioned in the books, and I don't know if it is any good.
Certainly Grischuk should not play 1.d4 and loose all his teeth against Kramnik's Slav version of the Berlin wall (I think this line should be called the iron curtain). Ponomariov knows why.