Twice Olympic champion Gail Devers of the United States and Ukrainian former world champion Zhanna Block scraped through to the semi-finals of the Olympic women's 100 metres on Friday.

Sprint hurdles specialist Devers, 37, was a late replacement in the U.S. team for the now-banned Torri Edwards. She looked sluggish in the morning first round and trailed in fourth in 11.31 seconds in the opening second-round heat.

With only the first three sure to qualify for Saturday's semi-finals, Devers faced an anxious wait and squeezed through as the fourth next-fastest, the last qualifying spot on offer, by one hundredth of a second from Russian Irina Khabarova.

Block, who won the world 100 metres title in 2001 but has suffered fitness problems of late, was the third fastest loser after finishing fifth in her heat in 11.27.

As in the morning session, Belarussian Yuliya Nesterenko clocked the fastest time of the round, 10.99, and was once again the only athlete to dip under the 11-second barrier. She looks likely to be a strong contender in Saturday's final.

The event is wide open with a host of leading contenders absent. Olympic champion Marion Jones failed to qualify at the U.S. trials and the first two finishers at last year's world championships, Americans Kelli White and Edwards, are banned for doping.

Greek Katerina Thanou, the Olympic silver medallist in 2000, is also absent after withdrawing in the wake of the scandal over her and compatriot Costas Kenteris's missed dope test which has rocked the Athens Games.

French 30-year-old Christine Arron again looked highly impressive, winning Devers's heat after easing down to 11.10.

So too was American Lauryn Williams who won her heat in 11.03. She said: "If I get a start like that (in the final) god knows what will happen."

Williams's team mate, U.S. champion LaTasha Colander, qualified third in her heat in 11.20.

Jamaican-turned-Slovenian Merlene Ottey, competing in her seventh Olympics at the age of 44, followed home two of her former Caribbean team mates Sherone Simpson and Aleen Bailey to qualify third in her heat in 11.24.