Britain's Tracey Hallam caused an upset to beat 2000 silver medallist Camilla Martin in the women's badminton.
The 29-year-old, competing in her first Olympics, edged out the Dane 11-2 5-11 13-10 to reach the quarter-finals.
But Hallam's team-mate Kelly Morgan went down 11-6 11-8 to second seed Ning Zhang of China.
Mixed doubles pair Nathan Robertson and Gail Emms, Britain's main medal hopes in badminton, won their opening match in comfortable fashion.
Meanwhile Anthony Clark and Nathan Robertson won their first round men's doubles match against Thailand's Prapakamol Sudket and Ngernsrisuk Patapol, 15-5, 15-9.
Hallam will meet Dutch fourth seed Mia Audina, a silver medallist in Atlanta in 1996, in the last eight.
British number one Hallam dominated the first set against Martin but grew tense in the second, which allowed the seventh seed back into the match.
But in the final set, Hallam's fitness was the telling factor although she had to save three match points before securing victory.
Hallam said she was "thrilled" with the win but said she had felt confident from the beginning of the match.
"Even in the first few points of
the game I knew I was feeling good," said the Brition.
"Even at the change of ends, when I was down, I kept trying to think of it as
the start of a new set with everything to play for and not as though I was
coming to the end of the match.
"I have nothing to lose in the quarter-finals. I am the underdog again and so
I can just go out there and play my own game without any pressure."
As well as Martin, another of the favourites for the event, sixth seed Jun Jae-Youn, also lost.
The South Korean was beaten by Taipei's Cheng Shao-Chieh.
However, Chinese top seed Gong Ruina reached the last eight, overcoming a sluggish start to defeat Thailand's
Ponsana Salakjit 11-8 11-3.
Morgan led 8-6 in the second set but poor shot selection cost her a chance to take the match into a decider.
"Although I am disappointed, I know that I can compete with the world's
best players and will keep working at my game to make sure that I win games like
these," said the Welsh player.
Fourth seeds Robertson and Emms, who had a bye in their opening round, cruised into the last eight with a 15-11 15-4 win over Germany's Bjoern Siegemund and Nicol Pitro.
Chinese top seed Lin Dan suffered a shock first-round at the hands of Singapore's Ronald Susilo in the men's singles.
The world number one was beaten 15-12 15-10 by the
unseeded Japan Open champion.
Britain's Robert Blair and Natalie Munt were beaten 15-8 15-12 by eighth seeds Nova Widianto and Vita Marissa of Indonesia.
South Korean top seeds Kim Dong-Moon and
Ra Kyung-Min and Chinese number two seeds Zhang Jun and Gao Ling eased through to the quarter-finals.
Blair and Munt admitted they were disappointed to lose out in their last 16 match.
"We were up in the second game but we made some silly errors to let them
back," said Blair.
"It is very disappointing because we know that we can play better than that."