The opening day of the U.S. Open claimed its biggest upset victim so far in No. 4 seed Holger Rune. (More Tennis News)

Spaniard Roberto Carballes Baena, ranked 63rd, downed Rune 6-3, 4-6, 6-3, 6-2, on Monday for his first-ever victory over a top-10 player. Rune, a 20-year-old Dane who cracked the top 5 after reaching the quarterfinals of the French Open and Wimbledon, had complained before the match about being assigned to one of the outer courts, sarcastically tweeting a map of Flushing Meadows to help his fans find it.

“I just didn’t expect to play on that court,” Rune said afterward. “That’s obviously disappointing but not going to blame the court on the loss.”

On the women’s side, No. 1-ranked defending champion Iga Swiatek needed just 58 minutes to beat Swede Rebecca Peterson 6-0, 6-1, 19th-seeded Beatriz Haddad Maia took out 2017 champion Sloane Stephens 6-2, 5-7, 6-4, and fourth-seeded Elena Rybakina, last year’s Wimbledon champion, defeated Marta Kostyuk of Ukraine 6-2, 6-1.

Spaniard Rebeka Masarova upset eighth-seeded Maria Sakkari 6-4, 6-4, with the Greek complaining afterward she was bothered by the smell of marijuana.

“The smell, oh my gosh,” Sakkari said to the chair umpire in the first set. “It was weed.”

Tenth-seeded American Frances Tiafoe, a 25-year-old from Maryland who reached the semifinal at last year’s U.S. Open, had little trouble in a 6-2, 7-5, 6-1, victory over wild-card countryman Learner Tien.

Dominic Thiem, the 2020 champion who is unseeded this year, downed 25th-seeded Alexander Bublik of Kazakhstan 6-3, 6-2, 6-4. Thiem, an Austrian who had lost his previous seven Grand Slam matches, next faces American Ben Shelton, who defeated Argentine Pedro Cachin in four sets.

The longest match so far, four hours and 29 minutes, went to Zhizhen Zhang of China, who outlasted American J.J. Wolf 7-5, 7-5, 6-7 (5), 4-6, 6-3.

Still to come are matches with Coco Gauff on the women’s side, and Novak Djokovic on the men’s.

Gauff, a 19-year-old from Florida, faces Laura Siegemund of Germany, who won the women’s doubles title in New York three years ago. Gauff was the runner-up at the French Open in singles — losing to Swiatek — and doubles in 2022.

Wrapping things up in the main stadium will be 23-time Grand Slam champion Djokovic against Alexandre Muller of France, who is making his U.S. Open debut. It marks a return to the event for Djokovic, who missed it last year because he couldn’t travel to the United States as a foreigner who is not vaccinated against COVID-19. That rule has been dropped.

The first round is played over Monday and Tuesday. The tournament lasts two weeks and wraps up with the women’s singles final on Sept. 9, and the men’s singles final on Sept. 10.