Olympics Day 14: Sprinters race for relay gold

LONDON (CNN) -- The relay baton will be center stage Friday evening with two Olympic golds at stake, in the women's 4 x 100-meter and the men's 4 x 400-meter relay races.

The lineup for Team USA in the men's race is sure to change from the semifinal, when Manteo Mitchell remarkably found a way to finish his opening leg despite a broken fibula.

The United States will have to find a replacement for Mitchell -- who has told how he heard the snap of the bone breaking but kept on running through the pain -- as it challenges the Bahamas for the gold.

Great Britain, also in the final, can count on the enthusiastic backing of much of the 80,000-strong crowd as it seeks to get in the medals.

Allyson Felix and Carmelita Jeter, who took gold and bronze, respectively, in the women's 200 meters, are expected to be back in the lineup for Team USA in the women's 4 x 100-meter relay final.

The Americans' chief rivals will be Jamaica -- although both teams will also be focused on the all-important baton after both teams botched handoffs at Beijing.

The qualifying rounds of the men's 4 x 100-meter and the women's 4 x 400-meter relays also take place.

The Jamaican outfit, with star sprinters Usain Bolt and Yohan Blake making up half the team, will surely be hard to beat after its dominance in the 100-meter and 200-meter men's finals.

The 25-year-old Bolt on Thursday became the first man in Games history to defend both the 100- and 200-meter titles, which he won in such convincing style at the 2008 Olympics in Beijing.

Victory in the relay final Saturday would give Bolt his sixth Olympic gold medal.

As the Games approach their conclusion Sunday, 17 gold medals are up for grabs Friday in eight different sports.

In the Olympic Stadium, there are finals in the women's 1,500-meter and 5,000-meter races, as well as the women's hammer and the men's pole vault.

Gold in the men's open water 10-kilometer swimming marathon in the Serpentine, the lake in London's Hyde Park, went to Tunisian Oussama Mellouli. Germany's Thomas Lurz took silver and Canada's Richard Weinberger claimed bronze.

Mellouli's victory gave Tunisia its first gold medal of the 2012 London Games. He also took bronze in the pool last week in the men's 1,500-meter freestyle race.

Spectators lined the banks of the Serpentine to watch the swimmers complete the grueling race.

Both men and women compete in the BMX racing semifinals and finals Friday, with more drama promised after plenty of spills in the qualifying rounds.

Medics were called to carry one injured rider off the concrete track after a smash in one of the men's semifinal runs.

Winds off Weymouth, on England's south coast, remained light Friday, posing a challenge to the men and women battling for medals in sailing events.

Australia's Mathew Belcher and Malcolm Page took gold in the men's 470 sailing, with the silver medal going to Great Britain and bronze to Argentina. New Zealand claimed gold and Great Britain silver in the women's 470 race, with the Dutch taking bronze.

In men's basketball, Team USA faces veteran Argentina in what could be a tough semifinal matchup for the dominant Americans. The other semifinal pits Spain against Russia.

The U.S. women's basketball team has already made it through to the gold-medal final, to be played against France on Saturday.

The Netherlands face women's world champions Argentina in the women's field hockey final Friday evening. Team GB and New Zealand will play for bronze.

In men's boxing, defending superheavyweight champion Roberto Cammarelle of Italy will take on world No. 1 Magomedrasul Medzhidov, of Azerbaijan, to wrap up a night of semifinal fights.

Ireland's John Joe Nevin and Great Britain's Luke Campbell will battle for the gold in the bantam 56-kilogram event Saturday night after winning through their respective semifinals.

The International Olympic Committee excluded Italian athlete Alex Schwazer from the London Games on Friday after he tested positive for a banned substance, recombinant erythropoietin (rhEPO), which aids red blood cell production.

Schwazer, 27, had been due to compete Saturday in the men's 50-kilometer race walk.