Great Britain set an Olympic record to win a third straight men's team sprint gold with a shock victory against world champions New Zealand.
Philip Hindes, Jason Kenny - who claimed his fourth Olympic gold - and Callum Skinner won in 42.440 seconds.
It was Britain's fourth gold at Rio 2016, taking their overall tally to 15.
Earlier, Laura Trott, Joanna Rowsell Shand, Elinor Barker and Katie Archibald set a world record in the women's team pursuit.
"We were outsiders coming into the Games so it is great for it all to come together," said Kenny, who joins Sir Bradley Wiggins, Ben Ainslie, Matthew Pinsent and Paulo Radmilovic as four-time British Olympic champions.
Britain have dominated track cycling at the past two Olympics, winning eight out of the 10 gold medals at London 2012 and claiming seven in Beijing four years earlier.
And they started strongly on day one in Rio's Olympic Velodrome.
Despite winning the team sprint in Beijing and London, Britain were not considered one of the favourites for gold having failed to earn a podium place at any of the World Championships since 2012.
Six-time Olympic champion Sir Chris Hoy retired following the London Games, leaving 23-year-old Skinner to fill the 'third man' void.
Questions had been asked whether Skinner could step up after a series of unconvincing performances, but the Scot produced a stunning last lap to see Britain home by a tenth of a second.
New Zealand's Ethan Mitchell, Sam Webster and Eddie Dawkins - the same trio who won gold at this year's World Championships in London - finished in 42.542 seconds.
France's Gregory Bauge, Francois Pervis and Michael D'Almeida took bronze by beating Australia.
Kenny closes on British Olympic greats
Victory means only rower Sir Steve Redgrave and fellow cyclist Sir Chris Hoy have now won more gold medals for Great Britain than Jason Kenny.
The 28-year-old from Bolton won team sprint at Beijing 2008 and London 2012, along with the individual sprint four years ago.
He also races in the individual sprint and keirin in Brazil, with the now-retired Hoy believing Kenny can add more to his personal tally.
"Jason Kenny really looks in great form now and he has to be a favourite for the sprint," the 40-year-old Scot said on BBC One.
"The keirin is a bit of a lottery but he has to be in with a shout."
'No-one expected us to win gold'
Hindes, part of the London 2012-winning team alongside Kenny and Hoy, on his second Olympic gold medal: "It is unbelievable. No-one expected us to win an Olympic gold medal after our World Championships performance. But we always believed in each other and we stepped up as a team."
Kenny on beating the world champions: "The Kiwis went fast in the semi-finals and broke our Olympic record so to be honest I was relying on them to fall to bits. But they didn't and we went out and beat them. That's what makes me proud."
Skinner on winning his first Olympic gold: "It's not been an easy road. So to come here and be Olympic champion is incredible. We've been working so hard and it shows it pays off. I've been training day in day out to improve my start and keep up with these boys."
World record joy for GB women
The men's team sprint victory capped a stunning day for Great Britain, which started with Trott, Rowsell Shand, Barker and Archibald setting a world record in their qualifying ride.
Trott and Rowsell Shand were part of the trio who won gold in London, with Barker and Archibald joining them in the new format of four riders.
Going seventh out of the nine teams, they shaved more than four tenths of a second off the previous best of 4:13.683 set by Australia in 2015.
GB will now meet Canada, runners-up at the 2015 World Championships, on Saturday (19:34 BST) for a place in the gold-medal race.
In the men's team pursuit, Britain set the fastest time in qualifying after finishing just three-tenths of a second outside their own world record.
Ed Clancy, Steven Burke, Owain Doull and Sir Bradley Wiggins clocked 3:51.943, almost three and a half seconds clear of second-placed Denmark.
They will face New Zealand in the semi-final on Friday.
Clancy and Burke, along with Peter Kennaugh and Geraint Thomas, set the previous best of 3:51.659 at London 2012.
Chris Boardman, 1992 Olympic individual pursuit champion:
"What a poetic way to do it. Callum Skinner had all that pressure, he's the one who had to improve and he delivered a gold medal.
"What a first lap from Philip Hindes - the most consistent starter in the world.
"It will still be sinking in now. It was all about psychology and dealing with the pressure. We know who could deal with it now."
Sir Chris Hoy, six-time Olympic cycling champion:
"That was a fantastic performance from GB to break the world record in the women's team pursuit.
"The conditions are not quick in here. The environmental conditions are a bigger factor on times than the track surface."
'What a moment' - social media reaction
Subscribe to the BBC Sport newsletter to get our pick of news, features and video sent to your inbox.