Paris - Fourth seed Alexander Zverev was dumped out of the Paris Masters on Wednesday by Dutchman Robin Haase in a 3-6, 6-2, 6-2 second-round defeat.
The 20-year-old German lost his composure after easing through the first set, as his bid for a third Masters title of the season was abruptly ended at Bercy Arena.
It was the world No 43 Haase's first-ever win over a top-five player.
Zverev will next play at the World Tour Finals in London later this month, while Haase goes on to a last-16 tie against either Joao Sousa or 13th seed Juan Martin del Potro.
Zverev was put under pressure from the off as he was forced to save four break points in the opening game, but he broke in game four and served out to take a one-set lead.
The unseeded Haase started to mix up his game well at the start of the second set, though, and reeled off three straight games as Zverev became frustrated.
The 30-year-old continued to send winners past his opponent and broke serve again to force a decider.
Zverev, the youngest player to break into the world's top five since Novak Djokovic 10 years ago, continued to struggle to make any impact on the Haase serve as he fell 3-2 behind in the third set.
Haase grabbed the crucial break with a stroke of luck from a net cord, before completing one of the biggest victories of his career with a run of four games on the spin.
Home hope Lucas Pouille kept his slim hopes of qualifying for the World Tour Finals alive by seeing off Spaniard Feliciano Lopez 6-3, 6-4.
The 17th seed needs to at least reach the final at Bercy to stand a chance of taking one of the final two places for the year-ending eight-man event, but he was handed a boost when given Roger Federer's place in the draw after the Swiss second seed's withdrawal.
American John Isner, who lost in last year's final to Andy Murray, battled to a 7-6 (7/2), 6-7 (11/13) 6-3 victory over Argentinian Diego Schwartzman.
The ninth seed saw three match points come and go in a marathon second-set tie-break, but held his nerve to set up a third-round meeting with either Richard Gasquet or Grigor Dimitrov.
Sixteen-time Grand Slam champion Rafael Nadal can become the oldest year-end world No 1 in history later on Wednesday with a win over South Korean youngster Hyeon Chung.
Results from the ATP Paris Masters on Wednesday (x denotes seeding):
Robin Haase (NED) bt Alexander Zverev (GER x4) 3-6, 6-2, 6-2
John Isner (USA x9) bt Diego Schwartzman (ARG) 7-6 (7/2), 6-7 (11/13), 6-3
Lucas Pouille (FRA x17) bt Feliciano Lopez (ESP) 6-3, 6-4