When the other team picks Singed, Jax, Jayce, Udyr, or really anyone else with a reasonably weak early game or that is heavily reliant on AAs and is melee.
Irelia vs Nasus is slightly favored for Irelia. Jax vs Nasus is dead even. Udyr vs Nasus is dead even. Jayce vs Nasus favors Jayce at the start, but once you get enough armor Jayce can't touch you.
Singed vs. Nasus is heavily Nasus favored. The slow keeps singed from getting to you for fling/poison harass, or can let you chase him down, along with the passive life steal mitigating the damage from poison and the ult taking down HP, which is typically the only defense that singed gets.
Whatever you do, never play Nasus vs Olaf. You will feed unless he is an idiot.

Always play passively in lane until at least lvl 9, do not overextend, and try not to push (Often it is best to give up some CS on the first minion wave to ensure the other player pushes).

If you fall behind in Q farm due to them harassing you, do not panic, and try to make it up immediately, focus on not dying and make up for it later when you can (Nasus is still strong without a lot of Q farm).

Always build Philo stone. G/10 is essential for keeping up in gold, as you will be focusing on farming Q and will often miss other CS due to harass by the other player, which puts you down by a reasonable amount, depending on the strength of the harass.
Build as much move speed/hp/resists as you can. Damage is not important, as you can get that from SS farm.

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Originally Posted by Pker5256 Meh you can build pretty tanky and still do a lot of damage. jungle nasus is ok too, lotta' sustain and his W? Perfect ganking opportunities.

O.O 90% of champs have better sustain than nasus??? are you trolling us? nasus has a passive lifesteal that starts off at 14% his Q is a modified auto atk therefore you gain lifesteal while using it and his ult gives u bonus dmg and deals AOE dmg every second. nasus can beat any hero top lane cause hes just too god damn beast

Run lifesteal quints. Farm, Farm, Farm. Get some tanky items and a sheen. Dominate.

I've said it a million times in a million threads: AP Nasus is the only Nasus worth playing. His Q hits just as hard as Triforce Nasus when you get a Lich Bane on him, his ult damage gets pretty intimidating, as does his E. Play him mid, rush a Rylai's, farm your Q as normal, but level your E first. The ONLY mid that has beaten me when I've played this has been Swain, so far anyway.

Sorc Shoes, Rylai's, Abyssal, Sunfire, Liandry's, Zhonya's. Your ult keeps doing damage through Zhonya's, so you can just ult and eat about 12% of their hp/sec while being untargetable. It's ****ing good, is what I'm trying to say.

Nasus, much like Poppy, is LEGENDARY for his **** laning phase. He's a top laner for sure (except for the never-popular, sometimes-viable support Nasus), but your primary focus should be farming with your Q. He sets up ganks very well with Whither, his passive lifesteal keeps him in lane through the harass, and his ult can deal serious pain if used at the right time. However, his biggestasset is Siphon Strike, which you should always try to use for last hits until you can one plow through lanes like no ones business.

Nasus is just a beast and should be played as such. Just farm him up then steam roll the team