Caroline Wozniacki is through to the second round of the US Open after Magdalena Rybarikova was forced to retire in the final set.
Despite breezing through the first set 6-1, the Dane was put through her paces against Rybarikova, who took the second 6-3 before succumbing to injury in the third with Wozniacki up 2-0.
The Slovakian was struggled with her movement throughout and eventually had to withdraw with a back problem.
Wozniacki will meet Aliaksandra Sasnovich in the next round.
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