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 quarterfinal with another former No. 1, 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 quarterfinals 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 leveling 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 quarterfinals 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.

Wawrinka on song in

Saint Petersburg

Three-time Grand Slam champion Stan Wawrinka advanced to the quarterfinals of the St Petersburg Open on Wednesday with a win over Russian fourth seed Karen Khachanov in two tie-break sets.

The 33-year-old Swiss star Wawrinka, a wildcard entry, knocked off world No. 24 Khachanov 7-6 (12-10), 7-6 (7-1) and awaits the winner between defending champion Damir Dzumhur and Guido Pella.

"I'm really happy as the level of my play was really good today," Wawrinka said. "We both played really well, really aggressive.

"I was expecting a tough match and it was really tough. My level was great and I'm really happy with everything that took place on the court today."

Wawrinka, the former world No. 3, who is now down at 88 after a battle with injury, started confidently breaking his rival's serve early.

But the Swiss, who has been struggling to find his best form since returning to the ATP Tour after two operations on his left knee a year ago, suddenly lost his nerve to allow the local favourite to break twice in succession.

Khachanov, 22, was serving for the set but Wawrinka broke again to level and force a tie-break, closing it out at the sixth attempt having also saved two set points.

Wawrinka then raced through another tie-break to seal victory over Khachanov in the first meeting between the pair.

Fifth-seeded Spaniard Roberto Bautista Agut and Canada's Denis Shapovalov, the seventh seed, reached round two with respective wins over qualifiers Luca Vanni, 7-5, 6-2, and Adrian Menendez-Maceiras, 7-6 (7-3), 6-2.

Nishikori downs

Metz champion

Japan's Kei Nishikori returned to action for the first time since losing in the US Open semifinals with a 7-6 (7-4), 6-3 win over defending champion Peter Gojowczyk in the Metz second round Wednesday.

World No. 12 Nishikori defeated the 45th-ranked Gojowczyk in 94 minutes and will face either Ugo Humbert of France or Georgia's Nikoloz Basilashvili for a semifinal place.

Nishikori made the semifinals at the US Open earlier this month but was easily defeated in straight sets by eventual champion Novak Djokovic. — AFP