LONDON - Jamaican sprinter and world-record holder Usain Bolt is out to prove that lightning can strike twice on Thursday when he seeks to defend his 200 metres title, while the United States closes in on China at the head of the Olympic medals table.

On the 13th day of competition in London, Briton Nicola Adams became the first woman to win an Olympic boxing title in a thrilling flyweight bout against pre-fight favourite Ren Cancan of China at a packed ExCel arena.

The 29-year-old won easily on points in a four-round blizzard of punches, flooring her opponent once. All eyes now turn to the athletics track for the big drama of the 200, where Bolt is aiming to become the first man to win both the 100 and 200 at two successive Olympics and be crowned, unofficially at least, the greatest ever sprinter. The lanky 25-year-old has not shied away from good-natured mind games with his main rival and training partner Yohan Blake, who sprang a surprise by beating him over both distances in the Jamaican trials. "I told Yohan the 200m will be different because that is my pet event," Bolt said earlier this week. "I'm not going to let him beat me again." Elsewhere at the main stadium, world decathlon record holder Ashton Eaton of the United States had a comfortable lead over his compatriot Troy Hardee with just the javelin and 1,500m to come. Kenyan David Rudisha is clear favourite in the evening's 800m. World champion Christian Taylor and fellow American Will Claye are likely to battle for triple jump gold.

Women's world record holder and defending champion Barbora Spotakova is one of the world's most consistent javelin throwers but faces stiff competition from Russia's world champion Mariya Abakumova as well as Germany's Christina Obergfoell and South Africa's Sunette Viljoen.

Two golds on Thursday took Britain's total to 24, five better than Beijing in 2008 and their best performance since 1908, when London first hosted the Games. In addition to Adams, Charlotte Dujardin won the individual dressage event on her horse Valegro after a freestyle that celebrated British musical classic "Pomp and Circumstance".

The Americans captured seven of the dozen athletics medals available on Wednesday, with Aries Merritt rocketing to victory in the men's 110 hurdles and Allyson Felix striking gold at her third attempt in the 200. Brittney Reese registered only two out of six attempts in the long jump but her second leap of 7.12 metres was good enough to win the third U.S. gold of the night. Misty May-Treanor and Kerri Walsh picked up their third straight Olympic beach volleyball title. And on Thursday teenager Claressa Shields claimed the women's middleweight boxing title, lifting their haul of golds to 35, just one behind China at the top of the medals table. On the U.S. team website, the United States stands above China based on medals won. Overall they have 83 to China's 78. In the 4x400m relay, South African double-amputee Oscar Pistorius was denied the chance to run his team's third leg in qualifying when second-leg runner Ofentse Mogawane collided with Kenya's Vincent Mumo Kiilu, sending both crashing to the ground. South Africa were given a place in the final on appeal. Jamaica failed to qualify in the event after Jermaine Gonzales pulled up injured midway through his leg.

Over bumps and around steep banks of the BMX cycling track, there were bone-shaking wipeouts as well as comfortable winners in the men's qualifiers ahead of Friday's final. Australia's recovery from a slow start to the Games continued on Thursday with gold in the men's K4 1,000m canoeing final on Dorney Lake outside London. Other winners on the water included Hungary's Danuta Kozak, who added the K1 500m title to her medals cabinet, while Germans Peter Kretschmer and Kurt Kuschela snatched victory in the men's 1,000m canoe pair.