Alan Campbell of Britain upstaged a number of world and Olympic champions to qualify fastest for the semifinals of the men's single sculls on Tuesday, providing another boost to a host nation that is threatening to dominate the Olympic regatta.

Campbell crossed in 6 minutes, 52.10 seconds in the first quarterfinal to beat the times of fellow winners Mahe Drysdale of New Zealand, Lassi Karonen of Sweden and Ondrej Synek, the pre-regatta favorite from the Czech Republic.

Reigning two-time champion Olaf Tufte of Norway squeezed into the semifinals by placing third in the last quarterfinal, just 0.59 seconds ahead of fast-finishing Dane Henrik Stephansen. The single sculls is easily the most competitive of the 14 Olympic disciplines, with 33 participants having started out in the heats.

That included Hamadou Djibo Issaka of Niger, who crawled over the line to loud cheers in a time of 9:07.99 in a lower qualification race to open the fourth day of action.

Britain is favored to win three women's events _ and has gold-medal chances in several men's disciplines _ when finals begin Wednesday but Campbell wasn't expected to be one of them.

Competing under cloudy skies, 2010 world champion Synek was the second quickest in 6:53.32 and Drysdale _ a five-time world champion _ next fastest in 6:54.86 as he bids to complete his illustrious resume with an elusive Olympic gold.

In the women's single sculls, Kim Crow of Australia showed she has the energy to compete in two disciplines by qualifying quickest in the quarterfinals _ a day after reaching the final of the double sculls.

Crow is the only competitor doubling up on Dorney Lake, but she looked like she had plenty in reserve by posting 7:34.29, more than a second quicker than world champion Miroslava Knapkova of the Czech Republic.

Six-time world champion Ekaterina Karsten _ a 40-year-old Belarusian looking to medal at a sixth straight Olympic Games _ qualified for the semifinals but finished behind Fie Udby Erichsen of Denmark in the final race.

The defending champions in the men's double sculls failed to reach the final, after David Crawshay and Scott Brennan of Australia finished fifth in their six-boat first semifinal. Argentines Ariel Rosso and Cristian Rosso won the heat, with double world champion New Zealand and Italy also progressing.

Slovenia, the silver medalist from the 2004 Olympics in Athens, held on to win the second semifinal from Lithuania. Britain also advanced.