By Alastair Himmer
"I don't even remember I was 5-1 down," Zvonareva told reporters after overcoming a poor start against Czech Kvitova.
"I wasn't thinking about it, just trying to find my rhythm and I couldn't find it in the beginning.
"But slowly I got into the match and started making the shots I usually make and was able to turn it around."
Kvitova had reached the last four after Maria Sharapova twisted her ankle in Thursday's first set in a rematch of their Wimbledon final.
But her game suddenly fell apart after a blazing start against Zvonareva, who moved through the gears and brutally swatted her aside in brilliant sunshine.
Zvonareva tore through the first set tiebreak, a weak forehand volley into the net by Kvitova gift-wrapping it to the Russian 7-2.
The second set was embarrassingly one-sided, Kvitova appearing in a hurry to reach the locker room as she dumped another backhand into the net on match point.
"I don't know what happened," a dejected Kvitova said.
"If I knew what happened I would tell you. That first set was mine for sure."
Lacking in star quality after the spate of casualties, Tokyo's cavernous Ariake Colosseum was around half full when Radwanska finally put away third seed Azarenka.
"It was a really tough match," Radwanska said. "She put up a hard fight... I'm just glad I was able to pull it out in the third set."
(Editing by John O'Brien; To query or comment on this story email firstname.lastname@example.org)