Open boots + pots,
Ahri's core is WOTA and Rylai's. After that, you nearly always want a Rabbadon's for the AP. By then, you probably need some MR or defense, depending on who's fed and their team composition. Usually Banshee's Veil is a good choice. With that, rylai's and hextech you've got good defensive stats and sustain, plus your ult is an awesome escape.
At that point you've got...
At some point, if they have a lot of MR, you want to have gotten a void staff. If not, now's about the time when you'd do that because they WILL start stacking MR.
That leaves you with one more slot when you sell your other doran's. The game's probably over, Abyssal's probably going to fill that last slot, especially if you went GA as your defensive item.