Marta Kostyuk will advance past Sloane Stephens to claim the US Open second round prize and keep her run alive at Flushing Meadows. Kostyuk enters as WTA #11 and she already beat Stephens on August 18, 2026, 7:5, 7:5 on a hard court. She arrives off a first-round 6:4, 6:1 win over Storm Hunter in o…