London - With a historic sprint 'double-double' in the bag for Usain Bolt, Tirunesh Dibaba bids to provide another on Friday when gold in the women's 5 000 metres would mean she had won both the longest track races at successive Olympics.
The Ethiopian retained her 10 000m crown a week ago with a scintillating last lap, demonstrating that she was back to her best after years of injury.
Bolt's time of 19.32 seconds in winning the 200 on Thursday night equalled the fourth fastest as he eased down in the last 30 metres, sensing that he was not on course to break his own world record. Silver went to his main rival and training partner Yohan Blake and bronze to a third Jamaican, Warren Weir.
"After a rough season I came out here and did it," he said. "I thought the world record was possible. I guess I was fast but not fit enough. I could feel my back strain a little bit, so all I did was to keep my form."
The successful defence of both his 100 and 200 titles was unprecedented and confirmed him as the greatest sprinter of all time.
If she can win on Friday, Dibaba will be emulating the 5,000/10,000m double of Finnish man Lasse Viren in 1972/76.
Her main challenger in the 5,000 may be Kenya's Vivian Cheruiyot, who has played second fiddle to Dibaba for most of her career but herself won the 5,000 and 10,000 at last year's world championships.
Bolt's triumph capped a remarkable day when Kenya's David Rudisha broke his own 800m world record, Britain's Nicola Adams became the first woman to win an Olympic boxing title and the United States held off a Japanese fightback to win their third successive women's soccer gold.
Jamaica's women try to keep their country's Olympic sprint magic flowing in Friday's 4x100m relay.
Women's double 100m champion Shelly-Ann Fraser-Pryce and third-placed Veronica Campbell-Brown will seek to repeat Jamaica's Beijing triumph against a U.S. quartet featuring 100m silver medallist Carmelita Jeter and 200m winner Allyson Felix.
The Jamaicans will be missing from the men's 4x400 after failing to qualify but the United States, seeking an eighth successive win in the event, are not as strong as usual and face a real battle to keep their streak alive.
They would have no chance at all but for Manteo Mitchell, who astonishingly secured a place in the final for his team by running through the pain of a broken fibula bone in his leg.
Friday also sees medals on offer in the women's 1 500 metres, women's hammer and men's pole vault, while the heats of the men's 4x100 relay should give the fans plenty of excitement.
In sailing, Australia and Britain will be racing for gold, weather permitting, in the postponed men's two-hander 470 class.
World record in 800m
Thursday's sellout audience of 80 000 in the athletics stadium were given the rare treat of a world record in a middle distance final, as Kenya's Rudisha became the first man to break one minute 41 seconds in the 800m.
"I had no doubt about winning but I was waiting for perfect conditions to break the record," he said, suggesting he might go even faster later in the season if conditions were right.
World record holder Ashton Eaton of the United States won the decathlon, comfortably ahead of compatriot Trey Hardee, while Czech Barbora Spotakova claimed the women's javelin gold.
There was another American one-two in the triple jump, where world champion Christian Taylor produced this year's biggest leap to take gold ahead of Will Claye.
At Wembley, in the women's soccer final, two goals from midfielder Carli Lloyd ensured that the Americans avenged their defeat by Japan in last year's World Cup final. The 80,000 crowd was the biggest for a women's Olympic soccer match.
The U.S. also won their first women's water polo gold, beating Spain 8-5 in the final.
The medal tally as competition got started on Friday showed the United States on top with 39 golds ahead of China on 37.
Three golds for the host nation Britain on Thursday - one for Adams with the other two in taekwondo and individual equestrian dressage - kept them in third place.
With 25 golds, they have already gone six better than in Beijing in 2008, the best performace since 1908.
But Britain's men's field hockey hopes were dashed when the team were crushed 9-2 by the Netherlands in their semi-final. In the final on Saturday, the Dutch will play title holders Germany, who beat world champions Australia 4-2.
Saturday's women's basketball final will be contested by France and the U.S., who have not lost at the Games in 20 years, after they beat Russia and Australia respectively.
Austria's sports minister promised a shake-up of state support for competitors after the country so far failed to win a summer Olympic Games medal for the first time in half a century.
"The new law on promoting sports should ensure that Austria no longer produces Olympic tourists," Norbert Darabos told the Kurier newspaper, saying he was fed up with athletes who had performed poorly gushing about the atmosphere at the Games.