Novak Djokovic delivered a lesson in Grand Slam tenacity to Learner Tien in the first round of the U.S. Open on Sunday as he battled past the American teenager 6-1, 7-6 (7-3), 6-2 to launch his latest quest for a record 25th major title.

Reigning women's champion Aryna Sabalenka, meanwhile, opened her bid to become the first woman to claim back-to-back U.S. Open titles since Serena Williams completed a hat trick of wins in 2014, with 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.