Aryna Sabalenka launched the defence of her US Open crown with a battling victory on Sunday as American men's hopes Taylor Fritz and Blake Shelton advanced safely.

World number one Sabalenka, who is bidding to become the first woman to win back-to-back US Open titles since Serena Williams completed a hat-trick of wins in 2014, was made to work hard en route to a 7-5, 6-1 victory over Switzerland's Rebeka Masarova.

Masarova, ranked 108th in the world, pushed Sabalenka out of her comfort zone with a gutsy performance on the Billie Jean King National Tennis Center's main Arthur Ashe Stadium showcourt.

Sabalenka will face unseeded Russian Polina Kudermetova in the second round.

In other early action on Sunday, Kudermetova's sister Veronika was on the wrong end of one of the few early upset

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