Zverev suffered a surprise 6-7 (5-7), 2-6 defeat to a resurgent Lorenzo Musetti in his opening match.
Two days after his rise to the top of the ATP rankings, the Hamburg native suffered a bitter setback. The 2024 tournament winner missed the last 16 at the indoor Masters for only the second time in nine attempts after a disappointing performance. Before the match, Zverev had said the fatigue he felt during the tournaments in Asia was no longer an issue.
"It's incredible. I'm almost a little shocked by how good my level was against Sascha," Musetti said after his fourth win in six matches against Zverev. It was the first time since 2022 that a world No 1 had lost their first match after taking the top spot. Jan-Lennard Struff is now the only German player left in the field. He faces Argentina's Thiago Agustín Tirante later on Wednesday.
Alexander Zverev honoured by Nadal, Sinner and others
Set to finish the current season at the top of the ATP rankings, Zverev took to the court at the arena in the Paris suburb of Nanterre for a ceremony on Monday. The 29-year-old received a trophy and congratulations from prominent figures in a video message. Clay-court specialist Rafael Nadal and Zverev's predecessor Jannik Sinner were among those who had warm words for the German.
"It's a fantastic feeling, of course, just incredible to be world No 1," Zverev told Sky before facing Musetti, who had also knocked him out of the 2024 Olympic tournament in Paris. Musetti has slowly fought his way back up recently after a difficult year full of injuries, which he began as world No 8.
Musetti also started well against Zverev. He kept the contest even but, just when Zverev looked certain to take the first set in the tie-break, raised his level. Five points in a row gave the underdog a 1-0 lead in sets.
With titles in Paris and New York already this year, Zverev also wanted to win the Davis Cup with the German team at the end of November. He needed to respond, but the blow took its toll. Musetti broke early in the second set to put himself on course for victory.