Rafael Nadal fell to a shock defeat by 18-year-old Canadian Denis Shapovalov at the Rogers Cup in Montreal.
Wildcard Shapovalov came from a set down to beat top seed Nadal 3-6 6-4 7-6 (6-4) and become the youngest player to reach a Masters 1000 quarter-final.
Nadal's loss means Andy Murray remains world number one, but only until next week's Cincinnati Masters, where Nadal or Roger Federer will replace him.
Federer also progressed to the last eight, with victory over David Ferrer.
The Swiss world number three, 36, came from a set behind to beat the Spaniard 4-6 6-4 6-2 and next faces Roberto Bautista Agut, also of Spain, who defeated Frenchman Gael Monfils 4-6 7-6 (7-5) 7-6 (6-2).
World number 143 Shapovalov, the 2016 Wimbledon junior champion, next plays 29-year-old Frenchman Adrian Mannarino after his stunning win over Nadal.
"He's honestly the best player I've ever played in my life," said Shapovalov, the youngest man to beat a top-two player since the 17-year-old Nadal beat Federer in 2004.
"You could tell why he's won so many Grand Slams. His ball was just so heavy. He's such a warrior out there. So it's honestly, like a dream come true for me to beat a player like that."
Nadal, 30, must wait until Cincinnati next week to see if he can regain the number one ranking
"It is probably my worst loss of the year because I lost against a player with a lower ranking," said the Spaniard.
"At the same time, because of the opportunity I had here to come back to number one. Always tough, but the draw was not that impossible after all the matches that were happening. So it was an important mistake from me."
In the other quarter-finals, Germany's Alexander Zverev, who beat Australian Nick Kyrgios 6-4 6-3, will play South African Kevin Anderson, who saw off American Sam Querrey 6-4 6-1 in the last 16.
And Argentina's Diego Schwartzman will face Robin Haase of the Netherlands. Haase beat Bulgaria's Grigor Dimitrov 7-6 (6-3) 4-6 6-1, while Schwartzman defeated American Jared Donaldson 0-6 7-5 7-5.