Felix Auger-Aliassime left Rafa Nadal's title hopes at the ATP Finals in tatters as he beat the struggling Spaniard 6-3 6-4 in round-robin action at Turin's Pala Alpitour Stadium on Tuesday.
Nadal needs to win the year-end title for the first time in his illustrious career to have a chance of ending the season as world number one, but he faded badly after an encouraging start.
The 35-year-old was beaten by American Taylor Fritz in his opener on Sunday and is bottom of the group with one match remaining against Norway's Casper Ruud.
Nadal, the record 22-times Grand Slam champion, has now lost four successive ATP Tour matches for the first time since 2009.
Auger-Aliassime, now coached by Nadal's uncle and long-time mentor Toni who sat in his box, will face Fritz in his final group match with a chance of making the last four.
Nadal held the upper hand at times in the first set and twice threatened to break the serve of his Canadian opponent who was beaten by Ruud on his tournament debut on Sunday.
Serving at 3-4 Nadal led 40-0 but a couple of double faults and baseline errors allowed Auger-Aliassime to break.
Auger-Aliassime, who has won four titles this season, sealed the first set with a solid hold and moved a break ahead early in the second.
Nadal showed flashes of brilliance but could make no impression as he slumped to a defeat.