The Flash provides a gap closer and a escape, and that ignite will do further damage with the ult. Although you are starting with nothing but health pots some people might be frightened and the person who is playing Mordekaiser might take heal. If using mordekaiser before lvl 12 for flash or lvl 7 for ignite, take eatiher Heal and ghost or Heal and Teleport as for the sustain.
It might be possible for the mid to be a high burst Champion like Fizz or someone that can poke like Lux or a Skill shot person that can do both such as Gragas. If so try to dodge!

He doesn't need mp to cast spells
Huge damage with Q->E->R->Ignite
A lot of sustain after Hexteck Revolver because of the AoE Q, W, and E provides
Can push Tower really fast with the W
Cant be out traded

Lack of range and can be out poked by Lux
No CC on Mordekaiser
No Gap Closer
Long Cooldowns in early game

13 Health pots because the Sustain and that all spells are depended on HP.
Hexteck Revolver brings HUGE SUSTAIN! it can lead to Never going back unless for shopping :)
Rabadons for the Damage
Zhonyas Hourglass for the OH **** THE JUNGLER or any gank
Will of the Ancient because the Aura and the upg version of hexteck
Abyssal scepter for the Magic resist and decreasing magic resist AND The Ap.

Don't tower dive as often because no CC unless the jungler does. he Synergies perfectly with a jungler that has a CC or that can Tank such as Amumu and Allistar and/ or Shen for the Taunt
Beware that the Skills use health so don't go **** you and use your Q and E to poke a lot because then you are easier to be ganked with such low health.
It is Smart to always invade the jungler when the mid is going to get the blue buff. The only thing that should not be on cooldown when you are doing that is the ultimate. The burst damage with the ultimate provides heavy damage.