(Photo credit: Robert Deutsch-Imagn Images)

The first round of a major can be tricky for the world's No. 1 player and defending champion. Especially when it's the last major of the year in New York.

But Aryna Sabalenka made the proceedings look very routine in a 7-5, 6-1 win over Switzerland's Rebeka Masarova in her first match at the U.S. Open.

Since winning the U.S. Open 50 weeks ago, Sabalenka has dropped two major finals in Melbourne and Paris and lost in the Wimbledon semifinals.

'I learned a lot of tough lessons this season on the Grand Slams,' Sabalenka said. 'How important it is? Pretty important. I really hope for the best here at the U.S. Open.'

Masarova, ranked No. 108, did put up a battle in the first set, splitting the first 10 games. But the Belarussian held her nerve and serve to go up, 6-5, and broke Masarova to breathe easier.

Sabalena cruised in the second set in the first step to her title defense.

'I think it's a lot of pressure, definitely,' said Sabalenka. Maybe every time defending champions comes and they put so much pressure on themselves. But I feel like I'm experienced enough to just focus on myself and try to replicate that result.'

The first upset of the day occurred on the Grandstand court, as 20-year old Alexandra Eala became the first native of the Philippines to win a Grand Slam main draw match. She rallied from a 5-1 third-set deficit to defeat No. 14 seed Clara Tauson of Denmark, 6-3, 2-6, 7-6 (11).

In the deciding set, Tauson broke Eala in the third and fifth games and served for the match at 5-1. But, with the crowd exhorting her on, Eala countered and won five consecutive games. Tauson (12-0 edge in aces) would fend off a match point in the 12th game and send the match to a 10-point tie-breaker.

Eala, who knocked off three Grand Slam champions (Jelena Ostapenko, Madison Keys, Iga Swiatek) at the Miami Open in the spring, finally won the tie-breaker on her fifth attempt.

'To be Filipino is something I take so much pride in,' Eala said. 'I don't have a home tournament so to be able to have this community here at the U.S. Open, I'm so grateful they made me feel like I'm home.'

The other early upset occurred on Court 13, where Indonesia's Janice Tjen (ranked No. 149) knocked off No. 24 seed Russian Veronika Kudermentova, 6-4, 4-6, 6-4.

Canadian Leylah Fernandez and McCartney Kessler, seeded Nos. 31 and 32, respectively, won their matches in straight sets.

Other winners in the afternoon were Caty McNally, Great Britain's Emma Radacanu, Tereza Valentova and Marketa Vondrousova of the Czech Republic, Russia's Polina Kudermentova and Japan's Moyuka Uchijima.

--Field Level Media