Chang enjoyed by far the biggest win of her career against Safina
World number one and defending champion Dinara Safina made a shock second-round exit with defeat by Taiwanese qualifier Chang Kai-Chen at the Pan Pacific Open.
Second seed Venus Williams also crashed out in Tokyo, losing 7-6 (8-6) 7-5 to Russian Anastasia Pavlyuchenkova.
Both Safina of Russia and Williams of the United States had first-round byes.
Fifth seed Svetlana Kuznetsova lost 7-5 4-6 6-3 to Germany's Andrea Petkovic, but fellow Russian Maria Sharapova beat Francesca Schiavone 4-6 7-5 6-1.
The defeat comes as the latest blow to Safina, who has been under pressure since reaching the top of the rankings as she has yet to win a Grand Slam title.
"It's not an easy moment," said Safina, who double-faulted to drop serve when trying to close out the match in the third set and went on to lose 7-6 (7-5) 4-6 7-5.
"I had a lot of chances in the third set but I just let it go. The way I played the game at 5-4 was not right after her latest setback.
"It's so disappointing I didn't play the way I should have played. I never took her out of her comfort zone."
Chang, ranked 132nd, was delighted after winning a marathon match in two hours, 44 minutes.
"I never came into the match thinking I was going to lose badly," said the 18-year-old. "I just wanted to play tough.
I never came into the match thinking I was going to lose badly
"I was jumping up and down at the end thinking about my mum. She never watches me play but she watches the live scores, so I was wondering what she was thinking."
Pavlyuchenkova served for the first set at 5-4 against Williams only to lose her serve, but took the tie-break 8-6 and raced into a 3-1 lead in the second set.
She had points for a 4-1 advantage but was pegged back to 3-3, before eventually sealing the win in the 12th game by breaking Williams to 15.
Sharapova, who won the event in 2005, was a set down and struggling in the second but fought back to seal the win and advance to the second round.
"I thought I did a good job of hanging in there because she was playing at a high level, some of the best tennis she has played against me in all of our match-ups before," said Sharapova.
Local favourite Kimiko Date Krumm was unable to follow up her success in Seoul as she suffered a first-round defeat at the hands of Canada's Aleksandra Wozniak.
On Sunday, Date Krumm became the second oldest player to win a singles title on the WTA Tour in the Open era when she beat Spain's Anabel Medina Garrigues in the final of the Korea Open.
In other matches, there were straight-sets wins for seeds Agnieszka Radwanska, Samantha Stosur and Marion Bartoli.
Japanese veteran Ai Sugiyama, playing in her last-ever singles match, was forced to retire when trailing 6-0 2-1 to 13th seed Russian Nadia Petrova.
"I was looking forward to my final match, and I was able to bring myself mentally up to this point," said the 34-year-old. "It is very disappointing to end like this."