Chang was the top seeded casualty in early men's singles action at Ariake Colosseum. On the women's side, third seed American Kristina Brandi retired after suffering from dehydration, while fourth seed Fabiola Zuluaga of Colombia and seventh seed Paola Suarez of Argentina both lost in straight sets.
In the upset of the day, repeated assaults on the net by the unseeded Mamiit forced Chang into making uncharacteristic mistakes.
"I wasn't able to put away as many balls as I'd like today," he said afterward.
The world no. 34, who was the tournament runner-up in 1994, built an early lead with a patient baseline game, pushing the ball deep into the corners and waiting for his opponent to make mistakes.
He broke Mamiit in the sixth game to go up 4-2, hitting an angled passing shot that his compatriot was barely able to touch with his racket.
But the 24-year-old Mamiit came back to break Chang twice in the first set as the one-time world No. 2 seemed to lose his composure.
He was blanked in the decisive 12th game, losing the set with a wild return from a hard Mamiit serve.
Mamiit, currently ranked 130th, continued to rattle Chang in the second set with aggressive net play. He improved his record to 2-1 against Chang, who was making his 10th appearance in the Japan Open.
In other action Wednesday, top seed Gustavo Kuerten of Brazil wore down Chile's Nicolas Massu 7-6 (5) 6-0 in a match of big serves and booming strokes.
Coming off a trip to the quarterfinals at the Sydney Olympics, the 24-year-old world number 3 was confident enough to simply trade shots from baseline with the unseeded Massu.
The 21-year-old Chilean, who reached his first tournament final just this year, gave as good as he got for most of the first set but seemed to tire in the second, when he was broken three times and struggled to find his returns.
"I was quite happy with the way I was finishing the match," Kuerten said. "I was there trying to find a rhythm."
Other second-round winners included sixth seed Byron Black of Zimbabwe, who defeated Daniel Elsner of Germany 6-2, 6-1.
Black's younger brother Wayne beat American Taylor Dent 6-4, 6-2. Dent was playing a substitute for second seed Nicholas Kiefer, who retired from the tournament Tuesday with an injury to his right wrist.
Thirteenth seed Jonas Bjorkman of Sweden struggled past local favorite Takao Suzuki 3-6 6-1 6-1.
The 28-year-old Swede punished Suzuki's forays to the net with surgical strikes from the baseline, while his opponent hurt himself with unforced errors.
Fifteenth seed Harel Levy of Israel cruised past South Korea's Lee Hyung-taik 6-2, 6-4, and 12th seed Sjeng Schalken of the Netherlands downed Sergi Spain's Bruguera 6-4, 6-2.
Unseeded South Korean Yoon Yong-il continued his impressive run, ousting 14th seed David Prinosil of Germany 4-6, 7-5, 6-1.
American Eric Taino beat compatriot Vincent Spadea 6-3, 7-5.
In early women's singles action, the second-seeded American Amy Frazier needed to win two tiebreakers to down countrywoman Corina Morariu 7-6 (5), 7-6 (3).
But three other seeded players weren't so lucky.
American Tara Synder was leading Kristina Brandi 6-2, 6-5 when the third seed retired, complaining of dehydration.
Fourth seed Fabiola Zuluaga of Colombia was upset 6-4, 7-5 by Taiwan's Janet Lee, while seventh seed Paola Suarez of Argentina was knocked off by South African Joannette Kruger 7-6 (5), 7-6 (0).
Fifth seed Tamarine Tanasugarn of Thailand brushed aside American Lilia Osterloh of the United States, 6-3, 6-1.
In a match between unseeded opponents, Sylvia Plischke of Austria defeated Wynne Prakusya of Indonesia, 6-3, 6-7 (2), 6-3.
In first-round women's doubles play, the fourth-seeded pair of Rita Grande of Italy and Meghan Shaughnessy of United States beat Kim Eun-ha of South Korea and Joannette Kruger of South Africa, 6-2, 6-1.
(news.ch)