By Julian Linden

LONDON (Reuters) - Ranomi Kromowidjojo of the Netherlands set the fastest time in the heats of the women's 50 meters freestyle on Friday to stay on course to complete the sprint double at the London Olympics.

Back in the water after winning the 100 gold the previous night, Kromowidjojo exploded off the blocks and charged through her single lap in a time of 24.51 seconds to win her morning heat.

In an ominous warning to her rivals, Kromowidjojo said she had not slept as much as usual because of the excitement of her 100 win.

"I went to bed at 12 (midnight) and got up at 7(am)," she said.

"I was just thinking about gold, gold, gold. It's madness on my Twitter account."

Marleen Veldhuis finished second behind her team mate Kromowidjojo in the heats and second overall.

The top 16 advanced to Friday night's semi-final which will decided the eight finalists for Saturday's last session of swimming in London.

"This was the first step," said Veldhuis. "I think that everyone who should be in the semi-final is in the semi-final."

Despite the frantic nature of the splash and dash, where places are decided by tiny fractions and a simple mistake can be the difference between winning and losing, there were no major casualties.

Britain's Francesca Halsall was third fastest and Germany's Britta Steffen, the defending Olympic champion and world record holder, was fourth.

"There are six girls who could get a medal and we're all very close," Halsall said.

"If I get through to the final, I'm not going to let anyone beat me."

The 16 qualifiers included two sisters, Bronte and Cate Campbell of Australia, who raced in the same heat and almost finished dead-level.

Bronte, the younger of the siblings, qualified ninth after stopping the clock at 24.87, while Cate reached the wall in 24.94 to finish 10th.

"I have been racing her for years. She brings a bit of comfort and normality to it," Bronte said.

"She's like another swimmer but a bit louder."

Cate won a relay gold medal on the opening night before being struck down by gastro and said she was surprised she was able to post a good time.

"I've been lying on my back for the past two days. I was happy to get out and swim," she said.

"I'm giving it my best shot and if my best shot is good enough then it is, if it isn't I'm still an Olympic gold medalist."

(Editing by Greg Stutchbury)