NEW YORK (AFP) — Aryna Sabalenka launched the defense of her US Open crown with a victory on Sunday while Novak Djokovic made a successful start to his latest bid for a record 25th Grand Slam singles title.

World No. 1 Sabalenka, who is bidding to become the first woman to claim 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.

"Happy to get this win and happy to be in the second round. I feel like I didn't start my best in the first games, but then I found my rhythm," said Sabalenka, who will face unseeded Russian Polina Kudermetova in the second round.

While Sabalenka advanced safely, Danish 14th seed Clara Tauson tumbled out to Alexandra Eala.

Eala, 20, became the first player from the Philippines to win a Grand Slam singles match with a thrilling 6-3, 2-6, 7-6 (13/11) win, battling back from a 5-1 deficit in the final set to advance.

Another player from Southeast Asia — Indonesia's Janice Tjen — also bagged an upset, ousting Russian 24th seed Veronika Kudermetova 6-4, 4-6, 6-4 to move on.

"I feel proud to be able to do this for my country," said Tjen, the first Indonesian since Angelique Widjaja in 2004 to reach the main draw of a Grand Slam singles event.

Tjen, ranked 149th in the world, will face Britain's Emma Raducanu in the second round.

Raducanu, who made history at Flushing Meadows in 2021 by becoming the first qualifier to lift a Grand Slam title, clinched her first US Open match win since that memorable triumph with a 6-1, 6-2 drubbing of Japan's Ena Shibahara.

"I wanted to win a match here really bad," Raducanu said.

"It's been four years, and it's a very special tournament for me."

With men's defending champion Jannik Sinner and rival Carlos Alcaraz not opening their first-round campaigns until Monday and Tuesday, all eyes were on the veteran Serbian star Djokovic in Sunday's night session on Ashe.

The 38-year-old showed flashes of brilliance in a 6-1, 7-6 (7/3), 6-2 win against his unseeded 19-year-old American opponent Learner Tien.

Djokovic raced through the first set but was then forced to save a set point in the second after Tien grew in confidence.

A lengthy medical time out to treat what looked like a blister on his right foot helped revive Djokovic and he pulled away to wrap up victory in two hours and 25 minutes.

"Great to be back in New York," Djokovic said.

"I wish I had Learner Tien's age — when you come to the late 30s it's about learning how to preserve the energy for what matters.”

"I still have the flair, I still have the drive, and you guys give me the energy," he told the crowd. "Hopefully, I can keep it going."

Fourth seed Taylor Fritz and sixth seed Ben Shelton eased into the second round.

Fritz, last year's beaten finalist and a semi-finalist at Wimbledon last month, won 7-5, 6-2, 6-3 against unseeded compatriot Emilio Nava.

Shelton had a similarly comfortable 6-3, 6-2, 6-4 defeat of Peruvian qualifier Ignacio Buse.

Shelton and Fritz are spearheading US hopes of a first Grand Slam men's singles title since Andy Roddick's victory at the 2003 US Open.