LONDON - China's Tang Yi set the pace in the preliminary rounds of the women's 100 metres freestyle at the London Olympics on Wednesday.

China's Tang Yi swims in her women's 100m freestyle heat during the London 2012 Olympic Games at the Aquatics Centre August 1, 2012. [Photo/Agencies]

Tang swam a personal best time of 53.28 seconds to win her morning heat and finish first overall in the race to discover the fastest woman in water.

Australia's Melanie Schlanger, who won a relay gold medal on Saturday, won her heat in 53.50 to be second overall, just ahead of Denmark's Jeanette Ottesen Gray and Aliaksandra Herasimenia of Belarus, who dead-heated in last year's world championships final.

With the top 16 advancing to the evening semi-finals, there were no major casualties in the preliminaries although Australia's Cate Campbell and Sweden's Therese Alshammar both pulled out of their heats because of illness.

Campbell, who teamed up with Schlanger to win gold in the 4x100 freestyle final, withdrew because she was suffering a bout of gastro while Alshammar had a pinched nerve in her back.

Colorado teenager Missy Franklin qualified 10th, just two places behind her American team mate Jessica Hardy.

"I think I did a good job. I had a controlled easy swim, faster than I was expecting," Hardy said.

Germany's defending Olympic champion Britta Steffen scraped into the semi-finals as the 14th qualifier. The final will be held at London's Aquatic Centre on Thursday.

"I am happy to have the chance to swim again this afternoon," Steffen said.

"Perhaps I can get in a bit more rhythm then. I would love to get into the final, but I realise there are youngsters up and coming."