Alexander Zverev made a losing start to his reign as world No 1 on Wednesday as the two-time Grand Slam champion went down 7-6 (7/5), 6-2 to an inspired Lorenzo Musetti at the Paris Masters.
Playing his opening match in the round of 32 in the French capital, the German accumulated errors to tamely exit the tournament against the 24-year-old Italian.
Former world No 5 Musetti became the fifth man to defeat a player in his opening match as the world No 1.
But Zverev insisted that the pressure of being the top-ranked player was not the cause of his poor performance.
"Stress, I don't know if stress. I was a bit annoyed, to be honest," he told reporters.
"The last couple of days, because, you know, achieving the world No 1 status is something that I think every player dreams of.
"But then I feel like I'm the only world No 1 in tennis history that has to explain why it's okay for me to be world No 1, which I find quite difficult, because I think I deserve to be where I am, even though I lost today."
In the opening set, both players staved off break points in consecutive games midway through the frame, but it always looked destined to be decided by a tie-break.
The Hamburg native got ahead 5-2 in the breaker but Musetti rallied to string together a run of five consecutive points to take a one-set lead, sealing it with a huge serve out wide to Zverev's backhand.
The Italian was now brimming with confidence and after failing to convert a break opportunity in the first game of the second set, he made no mistake two games later as he capitalised on an error-strewn display from Zverev.
Musetti continued to deploy a sliced backhand to devastating effect as unforced errors mounted on the 2024 Paris Masters champion's side of the net, culminating in a second break which allowed the Italian to serve for the match.
The world number 24 did just that, sealing his progress to the last 16 to love against a dejected Zverev.
"To simplify it, he played really good and I played really bad," Zverev told reporters.
"I think this was not a very good match from me, I think it might be one of the worst in the last couple of months."