As the track and field events got underway, there was an astonishing new world record. Also, Angelique Kerber reached the tennis final and Michael Phelps was denied his 23rd gold after a shock defeat.
Ayana smashed one of the longest-held records in athletics, breaking the existing 10k record by 14 seconds.
“Crystal clear” Ayana obliterates 10k world record
Ethiopia's Almaz Ayana brushed off doping suspicions about her astonishing world record-breaking Olympic 10,000m victory, putting her performance down to hard work and religious devotion.
The 24-year-old smashed one of the longest-held records in athletics as she romped to victory in an electrifying start to the track and field competition.
Ayana's winning time of 29min 17.45sec sliced an astonishing 14 seconds off the previous world best of 29:31.78, set by Wang Junxia in 1993 during the era of notorious Chinese coach Ma Junren.
The astounding display immediately raised eyebrows in the athletics world, with British women's marathon world record holder Paula Radcliffe one of the first to remark upon the Ethiopian's performance.
"I'm not sure I can understand that," Radcliffe said. "When I saw the world record set in 1993 I couldn't believe what I was seeing. And Ayana has absolutely blitzed that time."
Sweden's Sarah Lahti, who finished 12th, more than two minutes behind, was also sceptical. “I do not really believe that she is 100 percent," Lahti was quoted by Swedish media as saying. "There is doubt."
Yet Ayana - who had only raced the 10,000m once before heading into the Olympics - smirked when asked to respond to suggestions that her performance might not be all that it seemed.
"My doping is my training, my doping is Jesus - otherwise I'm crystal clear."
Later in the evening, Michelle Carter of the United States stunned defending champion Valerie Adams with her very last throw to win the gold medal in the women's shot put final.
Kerber will go for gold
Australian Open champ Angelique Kerber will play for the Olympic singles gold medal after yet another big win in 2016.
The second-seeded German beat American Madison Keys 6-3, 7-5 in the semifinals Friday, saving all 10 break points she faced. Kerber will play Puerto Rico's Monica Puig for gold Saturday.
The seventh-seeded Keys will face two-time Wimbledon champ Petra Kvitova for the bronze medal on Saturday.
On the men's side, favourite Andy Murray, the world number two, edged into the last four with a 6-0 4-6 7-6(2) victory but was pushed to the limit by the 22nd-ranked Johnson.
The players traded service breaks and blazing passing shots in the final set but Murray showed steady nerves to win the tie-break, closing out the match with a deftly placed overhead smash to set up a semi-final against Japan's Kei Nishikori.
Rafael Nadal, currently ranked fifth in the world, was tested by No. 54-ranked Belucci in a centre court stadium filled with passionate Brazilian fans, but found his rhythm in the final two sets, to clinch the 2-6 6-4 6-2 victory.
"The atmosphere was tough, but at the same time I enjoy it," Nadal told reporters. "I managed to come back in some tough situations." Nadal will take on Argentina's Juan Martin del Potro.
Solo labels Sweden ‘a bunch of cowards'
USA goalkeeper Hope Solo has come in for heavy criticism after she laid into the Swedish team, who beat favourites and reigning Olympic football champions 4-3 on penalties.
The score was locked at 1-1 at 90 minutes after Alex Morgan had equalized with 13 minutes to go, but Sweden emerged victorious in the shootout – and Solo attacked Sweden for their defensive tactics.
“I think we showed a lot of heart. We came back from a goal down, I'm very proud of this team. And I also think we played a bunch of cowards. The best team did not win today, I strongly, firmly believe that,” Solo said.
“They didn't want to open play. They didn't want to pass the ball. They didn't want to play great soccer. It was a combative game, a physical game. And they tried to counter with longballs.”
Solo's comments came after she delayed the start of the penalty shootout in order to change her gloves.
Elsewhere, Germany defeated China 1-0 to make the last four. Melanie Behringer netted the winner 14 minutes from time, with China missing a late penalty.
Germany shoot up the medal table
Gold medals in the shooting and dressage have catapulted Germany up to sixth in the medal table.
Germany's Henri Junghänel claimed gold in the men's 50m Rifle Prone after the 28-year-old delivered a nerveless performance in the final, notching up the country's second shooting medal in as many days.
And a smooth, controlled display from the German dressage team saw them win the country's sixth gold in Rio. Isabell Werth, Kristina Sprehe, Dorothee Schneider and Sönke Rothenberger held off a British challenge to win.
Brits dominate cycling
Britain's cycling pursuit team set a new world record as they beat Australia in the gold medal race.
The victory saw Bradley Wiggins, the 2012 Tour de France winner, become the first British athlete to win eight Olympic medals, surpassing fellow cyclist Chris Hoy's tally of seven.
Wiggins, along with Ed Clancy, Owain Doull and Steven Burke, beat Australia by almost a second.
"It was gold or nothing. I'll be hungover tomorrow," said Wiggins. "I'm trying to soak it all up. It was a horrible race to be in. It's over in a flash. It's a relief. Eighteen months ago there were doubts about whether I could come back and do this."
In the women's event, Gong Jinjie and Zhong Tianshi claimed China's first ever Olympic track cycling gold medal, beating Russian pair Anastasia Voinova and Daria Shmeleva in the velodrome.
The gold medal followed a new world record set in semi-finals earlier on Friday, posting a time of 31.928sec to surpass their 32.034 at last year's world championships in France.
The German team of Miriam Welte and Kristina Vogel, Olympic champions in London in 2012, beat the Australian pair of Anna Meares and Stephanie Morton to claim bronze.
Fraser leads golf competition after round two
Australia's Marcus Fraser followed his opening-round 63 with a 2-under 69 on Friday for a one-shot lead over Thomas Pieters of Belgium at the halfway point. He was at 10-under 132.
Henrik Stenson was two shots behind, and his second round did not lack of excitement. The British Open champion was caught in the worst of the rain early in his round, but the Swede escaped the bad conditions by making a 108-foot par putt. Stenson, who completed his round in 68, described it as the longest putt of his career.
Iran win first gold of Games
Weightlifter Kianoush Rostami set a new world record of 396 kilograms in his 85kg weight class, securing Iran their first medal and their first gold at the 2016 Rio Games.
Rostamihad set the previous record of 395kg himself in May in Iran. In Rio, China's Tian Tao also lifted 395kg and won silver.
Rostami set a new world record of 396kg in his 85kg weight class, securing Iran their first medal in Rio.
Female athlete Rim Jong Sim carried away the first gold medal for North Korea in Rio by winning in the 75kg class.
"I was really overwhelmed with happiness that I won this gold medal so I can send this honour to my leader," she said, referring North Korean leader Kim Jong Un.
Finnish weightlifter Milko Olavi Tokola did not win glory with his clean and jerk, but his faint and tumble off the stage guaranteed him attention.
"It hasn't happened for a while," Tokola said after regaining consciousness.
Russia claim another fencing gold
Russia's fencers stormed back against France in the closing minutes of the team foil final to clinch gold, while the US men beat defending champions Italy for bronze, their first medal in the event since 1932.
Trailing to France through each of their first seven bouts, the Russian team surged into the lead with one match-up left courtesy of Alexey Cheremisinov and Artur Akhmatkhuzin, who outscored the French 19-8 in just two of their bouts.
"It was the team spirit and we believed in ourselves," said Cheremisinov, who on Sunday lost his first match in the individual competition to teammate Timur Safin.
Russia has collected six fencing medals, three gold, so far in Rio.
Phelps denied, Ledecky flies
Michael Phelps suffered a shock defeat in the pool as Singapore's Joseph Schooling beat the American in the 100m butterfly.
Twenty-one-year-old Schooling produced an Olympic record 50.39 to take gold and force Phelps into silver, which he shared with both Chad le Clos and Laszlo Cseh in 51.14.
Phelps was searching for a fifth gold of the Rio Games, a 14th individual Olympic title and a staggering 23rd gold of his glittering career.
Phelps’ compatriot Katie Ledecky had a more productive evening, making history as she completed the first 200m, 400m and 800m treble since 1968 - and in world-record time.
The 19-year-old won the 800m by a distance in a time of 8min 04.79sec, beating her own world record of 8:06.68 set in January.