AK hand on PokerStars

[M.B.E.]

This is from a 4-player winner-take-all heads-up NLHE tournament. (Buyin $52.50, winner gets $200.) I won the first round and this was the fourth hand of the second round.

I have a small chip lead, 3210 to 2790. Blinds are 20/40 and I have the small blind on the button. I am dealt A[img]/images/graemlins/diamond.gif[/img]K[img]/images/graemlins/diamond.gif[/img] and just call. My opponent checks his option. 80 in pot.

FLOP: J[img]/images/graemlins/spade.gif[/img] 9[img]/images/graemlins/spade.gif[/img] 7[img]/images/graemlins/club.gif[/img]

My opponent checks, I bet 90, my opponent calls. Now 260 in pot.

TURN: [J[img]/images/graemlins/spade.gif[/img] 9[img]/images/graemlins/spade.gif[/img] 7[img]/images/graemlins/club.gif[/img]] A[img]/images/graemlins/heart.gif[/img]

My opponent checks, I bet 140, my opponent calls. Now 540 in pot.

RIVER: [J[img]/images/graemlins/spade.gif[/img] 9[img]/images/graemlins/spade.gif[/img] 7[img]/images/graemlins/club.gif[/img] A[img]/images/graemlins/heart.gif[/img]] J[img]/images/graemlins/club.gif[/img]

Now my opponent pushes all in. This seems really odd -- as I mentioned this is only the fourth hand I've played against him, and I certainly haven't seen him go all in before. The pot size is only 540, and he's betting all his chips, 2520. Surely if he had a great hand and wanted me to call, he would bet less. Or maybe that's just what he wants me to think.

If I call and my AK is good, then I'll have won the whole tournament. If I call and lose, my opponent's chip lead will be 5580 to 420. What should I do?

Results later.