Tokyo: Italian Camila Giorgi dashed Caroline Wozniacki's hopes of a winning a third consecutive Pan Pacific Open title with a stunning ambush of the top seed on Thursday.
Giorgi, ranked 35 places below Wozniacki at 37th in the world, completed a 6-2, 2-6, 6-4 upset in Tokyo to set up a quarter-final with another former number one, Victoria Azarenka.
Azarenka thrashed Australia's Ashleigh Barty 6-4, 6-2 as the Belarusian seeks a return to the top echelons of women's tennis after becoming a mum in late 2016.
Giorgi reached the Wimbledon quarter-finals earlier this year and showed exactly why as she took just 38 minutes to take the first set with some brutal hitting.
The Italian sealed the advantage with a looping forehand that nibbled the back edge of the baseline, wrong-footing Wozniacki, who cut a forlorn figure as she trudged back to her seat shaking her head.
Wozniacki hit back to take the second set, the Australian Open champion levelling with a ferocious backhand that forced Giorgi into a wild shot.
But Giorgi dug in and broke for 4-3 in the decider with another murderous forehand before delivering the coup de grace in a little over two hours with a fizzing backhand that Wozniacki could only waft into the net.
"It was a good match, nice court, it was fun," said Giorgi, who will hope for more of the same against Azarenka on Friday. "I'll just try to focus on my game plan as always and hope to play very consistently, like today."
Azarenka, who has tumbled down the rankings following a lengthy custody battle over baby Leo, will be the fresher of the two players.
Currently ranked 63rd and competing as a wildcard, she clinched the opening set with a thumping pass that was too hot for Barty.
Azarenka raced to a 4-1 lead in the second and closed out proceedings by jamming up Barty with a vicious kick serve into the ribcage on her second match point.
"That makes me feel a little old," said the 29-year-old when informed she had reached the quarter-finals for the first time since 2012.
"I'm just trying to look day by day and see what happens."
Czech Barbora Strycova won a tempestuous encounter with Estonian Anett Kontaveit 7-6, 3-6, 7-5 and next faces Japan's newly crowned US Open champion Naomi Osaka.
Eighth seed Strycova took the first-set tiebreak 7-5 but both players screamed in frustration at their wastefulness in a contest that lasted well over three hours.
Having used all her challenges, Strycova huffily staged a sit-down protest after Kontaveit saved one of eight match points with a passing shot that replays showed missed the line by some margin.
But the Czech returned to her task and closed out the match when Kontaveit whiffed a drive-volley long.
Elsewhere, Croatian Donna Vekic beat Britain's Johanna Konta 6-3, 7-5 to advance to the last eight, where she faces second seed Caroline Garcia of France.