The Speed Force gives Flash hax abilities that even Superman can't match — but the win condition is debatable.
Superman and Flash have raced many times in DC comics, and the results vary. When both are serious, Flash is faster — the Speed Force is explicitly beyond anything Kryptonian super-speed can match.
But speed isn't the only factor in a fight. Superman's strength and durability far exceed Flash's. Barry needs the Infinite Mass Punch to generate enough force to hurt Superman, while Superman can hurt Flash with his base strength.
Flash's Speed Force abilities — phasing, speed-stealing, time manipulation — provide win conditions that bypass raw power. He could potentially steal Superman's speed (leaving him a normal Kryptonian), vibrate through his body, or trap him in the Speed Force.
The question is whether Flash can use these abilities against a friend who is also incredibly fast and has heat vision that tracks targets at lightspeed.
Speed Force > Kryptonian speed, confirmed by DC canon. Phasing and speed-stealing bypass Superman's invulnerability. The Infinite Mass Punch generates enough force to hurt even Kryptonian biology.
Superman's strength and durability dwarf Flash's. One punch incapacitates Barry. Heat vision tracks at lightspeed. Superman doesn't tire while Flash's Speed Force use has practical limits in sustained combat.
Flash takes a narrow edge through Speed Force hax. He's confirmed faster and has abilities that bypass physical defense. But Superman's one-punch knockout potential makes every exchange dangerous for Barry.
The Flash also fights
Superman also fights