Coco Gauff survived a scare before going past Magda Linette with a 5-7, 6-3, 6-4 victory at the US Open.
Andy Murray lost in the first round of the US Open 2021 with a five-set loss to Stefanos Tsitsipas.
Murray went down gracefully as one of the hardest and longest matches in the history of Grand Slam tournament.
Day 1 also features second seed Daniil Medvedev and eighth seed Casper Ruud against French veterans Richard Gasquet and Jo-Wilfried Tsonga, respectively.
Novak Djokovic is seeded number 1 in the US Open and, unsurprisingly, is the strongest favourite to win the crown at the Flushing Meadows.
Russian sensation Rublev registered his maiden victory over his compatriot to reach the final of the Western & Southern Open.
The German overcame a double break in the third set to beat Greek star Stefanos Tsitsipas in the semifinal of the Western & Southern Open.
The Greek star squandered two match points in the second set but still managed to earn a spot in the last four in Cincinnati.
Naomi Osaka was stunned 3-6, 6-3, 6-3 by wild card entry Jil Teichmann in the Western & Southern Open's third round in Cincinnati.
The scoreline at the end of the match was 6-2, 6-2 in favour of the Russian and he will now face another American, Reilly Opelka in the final