Pegula was supposed to fly to Scotland in order to represent the US in the Billie Jean King Cup Finals in Glasgow. She won't because she withdrew from the event after playing a lot of tennis in the past few days at the WTA Finals.
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She went 0-3 in singles and 0-3 in doubles for a combined record of 0-6 at the event. There was no reason specified but it's probably related to fatigue as she did not look injured at the WTA Finals. She did briefly comment on it on Twitter:
"Really upset I couldn’t represent the USA in the finals. Wishing all the girls the best of luck in Glasgow!!"