Speed Force hax let Flash run circles around the thunder god — but one Mjolnir hit could end it.
Flash's speed is beyond anything Thor can match or react to. At full Speed Force power, Flash can steal Thor's speed, phase through Mjolnir, and deliver Infinite Mass Punches faster than Asgardian reflexes can process.
But Thor's durability is godlike, and a single hit from Mjolnir carries enchanted force that could potentially drop Barry. Flash needs to ensure Thor never lands a hit — and Mjolnir has homing capability of its own.
The key factor: Flash's hax abilities (speed-stealing, phasing, time manipulation) provide win conditions that bypass Thor's raw power. Thor can't fight what he literally can't perceive.
Speed Force supremacy makes Flash untouchable. Speed-stealing could leave Thor powerless. Phasing negates Mjolnir. Infinite Mass Punch generates enough force to threaten even Asgardian durability.
One Mjolnir hit ends Flash. Thor's lightning is area-of-effect and hard to dodge. Mjolnir has its own homing capability. Thor's godlike durability means even Infinite Mass Punches might not be enough.
Flash takes the edge through Speed Force hax. Thor's power is enormous but irrelevant if he can't land a hit, and the Speed Force provides tools that bypass physical defense.
The Flash also fights
Thor (Norse) also fights