Death Link makes it a bit confusing to resolve the upkeep effect.
It creates two separate effects on two different creatures (the warlock and the enemy creature) - it heals the warlock and only then deals damage to the creature according to the amount healed.
If I understand it correctly, then the result depends on who is has initiative.
If the warlock has initiative:
1) Warlock at 1 hp, enemy creature at 1 hp
2) Death Link resolves: Warlock at 3 hp, enemy creature dies
3) IoP resolves: Warlock at 2 hp
If the opponent has initiative:
1) Warlock at 1 hp, enemy creature at 1 hp
2) IoP resolves: Warlock dies, enemy creature dies