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Dagmara Wozniak faced Aleksandra Socha in her opening bout, outscoring her 5-1 to set the tone early for the U.S.
The United States led by five but quickly found themselves in danger of losing in the closing period.
Socha was back on the piste against Mariel Zagunis and scored three consecutive points to open the period.
Socha eventually tied the bout at 41 as Zagunis struggled to create any offense.
Zagunis regained a two point advantage only to see Socha tie on a questionable call. It had appeared Zagunis had the right of way on the simultaneous touch as Socha had stopped short but the point was awarded to the Polish fencer.
Zagunis closed out the bout with two consecutive touches to earn a trip to the semifinals against Russia.
Russia dominated their match against the eighth seeded Mexico, outscoring them 40-19 through the eighth period. Julieta Toledo did earn an impressive 12 point win over Sofya Velikaya in the ninth but it wasn't enough as Russia handily won 45-31.
Italy led for the majority of the match but had a scare in the sixth as France's Charlotte Lembach outscored Rossella Gregorio 8-5 to put France within two points of tying. Italy went on to win the last three bouts, taking a 45-36 victory.
The countries traded leads consistently throughout the close match. The Ukraine held a two point advantage into the ninth where number two ranked fencer Olga Kharlan closed out the bout for them with a 5-2 victory to advance to the semifinals