Alexander Zverev's tenure as the No. 1 player in men's tennis started with a 7-6 (5), 6-2 loss to Lorenzo Musetti in the second round of the Paris Masters on Wednesday.
Zverev moved to the top ranking on Monday, capping off a fine season that saw him win the first two Grand Slam titles of his career at the French Open and US Open.
Despite playing on a fast indoor court that suits his serve and aggressive ground strokes, the imposing German looked out of sorts against Musetti. The mercurial Italian clinched victory with an ace in a one-sided contest where he had more winners (29-25) and less unforced errors (21-12) than Zverev.
"It's pretty shocking the level I was able to play in front of [Zverev], who was serving really well, especially in the first set," Musetti said in his on-court interview. "It was really close. It could have gone both ways. The key was to stay there at 5-2. I played five great points in a row."
Also Wednesday, 24-time Grand Slam champion Novak Djokovic was facing qualifier Rinky Hijikata and seven-time major winner Carlos Alcaraz played American Alex Michelsen. Fourth-seeded Daniil Medvedev took on Ignacio Buse.
Djokovic needs to reach the semifinals to stand a chance of qualifying for the season-ending ATP Finals later this month. Only two spots remain for the Nov. 15-22 event in Turin, with only an ATP 250 event remaining in Stockholm after Paris.
In other second-round play, three-time US Open semifinalist Frances Tiafoe wasted four match points in a 6-3, 1-6, 7-6 (9) loss to Frenchman Quentin Halys.
Tiafoe's defeat qualified Arthur Fils for the ATP Finals. He is the first Frenchman to reach the event since Gael Monfils in 2016.
No. 8 Alex de Minaur and No. 15 Learner Tien both lost in straight sets, while No. 12 Tommy Paul advanced 6-1, 6-1 over Daniel Merida.
The Associated Press and PA contributed to this report.