The unseeded duo Maeda and Suetsuna took advantage of unforced errors by the top-seeded Chinese pair en route to an 8-21, 23-21, 21-14 win at Beijing University of Technology Gymnasium.

After they won, both Maeda and Suetsuna bowed deeply in the center of the court.

"This is the biggest win of our lives," Suetsuna said.

"They are the Olympic champions so even when we had match point we couldn't take anything for granted."

It was the first major setback of the tournament for the host nation, which is looking to dominate on home soil after winning three of five gold medals in Athens.

"We couldn't focus in the third set," said Yang. "We just made too many unforced errors."

Japan's Kumiko Ogura and Reiko Shiota, scheduled to play later on Thursday, are sixth in the world rankings and very popular in Japan.

Few had expected such a strong showing from Maeda and Suetsuna, who are eighth in the rankings.

Earlier, Malaysia's Lee Chong Wei defeated Singapore's Ronald Susilo in the second round of men's singles.

Second-seeded Lee, who is bidding for Malaysia's first Olympic gold medal, downed Susilo 21-13, 21-14.

"I was nervous at first but got more confidence as the game went on," said Lee, a favorite to win the tournament.

"There is a lot of pressure on me here to get a medal."

Susilo upset Lin in the first round four years ago in Athens but was no match for Lee's powerful smashes on Thursday.

Top-seeded Lin Dan of China will play his first match later on Thursday, taking on unseeded Ng Wei of Hong Kong.

South Korea's Lee Hyun-il defeated fifth-seeded Kenneth Jonassen 15-21, 21-14, 21-19 while third-seeded Bao Chunlai of China defeated Guatemala's Kevin Gordon 21-17, 21-16.

Indonesia's Sony Dwi Kuncoro, who is seeded sixth, defeated Boonsak Ponsana of Thailand 21-16, 21-14.

In women's singles, Indonesia's unseeded Kristen Maria Yulianti upset sixth-seeded Tine Rasmussen of Denmark 18-21, 21-19, 21-14 to advance to the quarterfinals.

"I was a little nervous but it's the Olympics and everyone has to deal with that," said Rasmussen. "I didn't play my best today and she took advantage."