The three Top 10 players who reside in the second quarter of the US Open main draw had mixed results on Day 1 of the year's final Grand Slam. Elena Rybakina and Karolina Muchova rolled into the second round, while Maria Sakkari was knocked out by Rebeka Masarova.

No.4 seed Rybakina romped past Marta Kostyuk 6-2, 6-1 in their first-round clash on the Grandstand. Kostyuk had defeated Rybakina earlier in the year in Adelaide, but Rybakina got her revenge in 1 hour and 27 minutes.

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Kostyuk, one of the highest-ranked unseeded players at No.39, picked up the first two Top 10 wins of her career this summer. But she was prevented from getting another one by Rybakina, who went 5-for-12 on break points. Kostyuk only converted one of her five break points.

2022 Wimbledon champion Rybakina's career-best result at the US Open is a third-round showing in 2021. It is the only one of the four Grand Slams where Rybakina has not reached at least the quarterfinals.

Rebeka Masarova in action during the first round of the 2023 US Open.

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Earlier in the day, Masarova of Spain pulled off a major upset by stunning No.8 seed Sakkari of Greece 6-4, 6-4, earning her first Top 10 win in the process.

World No.71 Masarova had been 0-4 against Top 10 players coming into the match, but the powerful 24-year-old was able to break that duck with a 1-hour and 27-minute win over 2021 US Open semifinalist Sakkari.

Masarova was behind 4-1 in the first set, but she collected the next five games consecutively to take the one-set lead. The Spaniard was never broken in the second set as she eased to victory. Masarova held three break points in the match and converted them all.

Masarova will play Anna Karolina Schmiedlova of Slovakia in the second round.

Sakkari has been a Top 8 seed at the last eight majors, but she has not won a match at a Grand Slam since this year's Australian Open. Sakkari lost in the first round of this year's Roland Garros to eventual finalist Muchova, and she fell in the Wimbledon first round to Kostyuk.

The upset stymied a potential all-Top 10 Round of 16 clash between Sakkari and Muchova. Muchova has already beaten Sakkari twice this season.

No.10 seed Muchova of the Czech Republic was the very first winner of the 2023 US Open. The newest member of the Top 10 needed just 65 minutes to oust Australian wild card Storm Hunter 6-4, 6-0 and clinch a second-round berth.

Muchova was up a break two times early in the first set, but Hunter fought back on both occasions to keep pace through 4-4. But Muchova turned up the heat from there, reeling off eight games in a row to claim her first US Open victory since her career-best run to the Round of 16 in 2020.

Oft-injured Muchova had fallen from the Top 20 to No.235 at this time last year, and she lost in the first round of the 2022 US Open to eventual quarterfinalist Ajla Tomljanovic.

Since then, the Czech has had a meteoric resurgence, making her first Grand Slam final at 2023 Roland Garros and her first WTA 1000 final earlier this month in Cincinnati. Muchova clinched her Top 10 debut two weeks ago following her Cincinnati run.

Muchova will play Magdalena Frech of Poland in the second round.