Kvitova crashed to a 6-4, 6-4 defeat to the Croat. Kvitova received a first-round bye.
The fourth seed Petra Kvitova became the biggest casualty of the second round at the Pan Pacific Open on Tuesday as she fell victim to the unheralded Petra Martic.
The 2011 Wimbledon champion from the Czech Republic, ranked fifth in the world against Martic s 73rd, crashed to a 6-4, 6-4 defeat to the Croat. Kvitova received a first-round bye.
Kvitova, who double-faulted three times to lose the opening game, again lost the first game of the second set to love and was never able to overcome an uphill battle throughout the 88-minute upset.
Martic, 21, who reached the fourth round at the French Open, said: "It was my biggest win so far. I beat a top-10 player at Roland Garros this year, but this is a very big win. I m really happy about this match.
"The last five or six days were really weird. My practices were really bad, I was struggling, I had no confidence."
Martic lost the first set of her first-round match in Tokyo 6-0, before recovering to set up the unlikely victory over Kvitova.
Last year s French Open champion Li Na of China, seeded seventh, needed more than two and a half hours to beat Anastasia Pavlyuchenkova of Russia 7-5, 4-6, 6-2, while ninth seed Marion Bartoli of France defeated Germany s Julia Goerges 6-2, 7-5.
Nadia Petrova of Russia reached the third round beating Romania s Simona Halep 7-6 (4), 6-0.