Defending champion Caroline Wozniacki flattened world number one Garbine Muguruza 6-2, 6-0 on Saturday to reach her fourth Pan Pacific Open final in Tokyo.

The Danish former number one will face Anastasia Pavlyuchenkova as she chases a third Tokyo title after the Russian toppled Angelique Kerber 6-0, 6-7, 6-4 in a see-saw first semi-final.

Top seed Muguruza, playing her first tournament since reaching the top of the women's world rankings, was blown off court in just under an hour in a stunningly lopsided match.

"I didn't feel that fresh," the Spaniard told reporters.

"It was always going to be a battle but I felt my energy was a little bit low," added Muguruza. 

"I was struggling a bit in the long rallies and didn't make the important shots. I'm very disappointed, but she just played better."

Third seed Wozniacki tore through the first set, sealing it when Muguruza wafted a routine forehand badly wide.

Muguruza cut a forlorn figure as she trudged back to her seat but things went from bad to worse for the Wimbledon champion in the second set.

A dipping forehand pass gave Wozniacki an early break, consolidated after another wild Muguruza backhand in the fourth game.

Wozniacki, a two-time Tokyo winner and finalist in 2014, put Muguruza out of her misery by forcing the Spaniard into another whiffed backhand on match point after just 59 minutes.

"Obviously you don't go on court thinking that's going to be the scoreline," said Wozniacki. "But I just kept my head down and stayed aggressive."

Seventh seed Kerber, another former world number one, looked in danger of being fed the dreaded "double bagel" after losing the first eight games in the Tokyo sunshine.

But the German recovered from 6-0, 5-2 down to pinch the second-set tiebreak as Pavlyuchenkova suddenly tightened up, the Russian slamming her racquet to the ground in disgust.

Kerber, who has slipped back to 14th in the world since winning last year's Australian and US Open titles, clinched the breaker 7-4 with a fizzing serve down the middle which she celebrated with a little fist pump.

The 29-year-old Kerber, a former Tokyo finalist, then quickly raced to a 3-0 lead in the deciding set, only to be pegged back once more.

But Pavlyuchenkova, who beat Wozniacki in the final at Monterrey earlier this year, snatched the decisive break in the seventh game with a superb backhand drive volley.

The world number 23 unleashed the same brutal shot on match point to seal victory after a little over two hours.

"I was already thinking about going to Wuhan," said Pavlyuchenkova, referring to next week's tournament in China.

"I was already booking flights in my head."