Rek'Sai's passive is great at sustain. Rek'sai's e has built in true damage for tanky top laners and Rek'Sai has great tower damage with her auto reset with her q. Also her borrow'ed q is free poke in lane. Also Rek'Sai's reduced vision in borrow'ed form can sense junglers if nearby, to avoid ganks.

Rek'Sai Top is very strong early game so you wanna look for 6 and pre 6 all ins. Your laning ability combos will be your borrowed q and then you tunnel to the enemy knock them up hit them with your q's 3 enhance auto attacks and when at max fury e them for true damage and finish the enemy off with your r that is a execute.

Rek'Sai is great for catching enemy champions out. Your knock up is a great set up for your teammate's. Use your R to go untargetable and help finish off marked enemy's that try to get away.