NEW YORK -- The opening day of the US Open claimed its biggest upset victim so far in No. 4 seed Holger Rune.
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 posting a map of Flushing Meadows on X, formerly known as Twitter, 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."
Rune suffered early exits from tuneup events in Toronto and Cincinnati and there were early signs of trouble Monday as he failed to earn a break point in the first set.
Carballes Baena converted on break point chances in the fifth and ninth games, but his serve lost its firepower in the second set, when Rune finally broke in the third game.
Carballes Baena, who won in Marrakech earlier this year, cleaned up his act in the third set, when he broke Rune to love in the eighth game, and had clear control over the match in the fourth set, when he dropped only a single first-serve point.
Also on Monday, 10th-seeded American Frances Tiafoe, a 25-year-old from Maryland who reached the semifinal at last year's US 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. He next faces American Ben Shelton, who defeated Argentine Pedro Cachin in four sets.
Thiem, who's had a series of injuries, hadn't won a Grand Slam match since the 2021 Australian Open, dropping seven in a row until Monday.
"It's a pretty special victory. ... Especially here at the US Open," Thiem said, "with all the past and all the memories I have here."
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.
Norwegian fifth seed and 2022 US Open runner-up Casper Ruud claimed a 7-6 (5), 3-6, 6-4, 7-6 (5) win over American Emilio Nava.
Wrapping things up in the main stadium was 23-time Grand Slam champion Novak Djokovic, who made a winning return to the US Open with a 6-0, 6-2, 6-3 victory over France's Alexandre Mullerin the evening session.
Djokovic missed the tournament last year, not allowed to travel to the U.S. because he is not vaccinated against COVID-19. It was his first match in Flushing Meadows since falling to Daniil Medvedev in the 2021 final, a loss that prevented the Serbian from completing a calendar-year Grand Slam in men's tennis since 1969.
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.
The Associated Press and Reuters contributed to this report.