Lizzy Yarnold overcame dizzy spells to win a second successive women's skeleton gold medal and become the first Briton to defend a Winter Olympics title.
The 29-year-old Sochi 2014 champion trailed leader Janine Flock of Austria entering the fourth and final run, where she overhauled the deficit to win by 0.45 seconds.
British team-mate Laura Deas claimed bronze by 0.02secs as Britain won two medals in the same event for the first time in Winter Olympics history.
Yarnold trailed by 0.10 overnight after complaining of being dizzy, but cut the deficit as overnight leader Jacqueline Loelling of Germany slipped back to third place after the third run.
The Briton trailed Flock by 0.02 ahead of the fourth and final run, meaning Yarnold was the penultimate slider to take to the track and had to watch her rival's run.
Yarnold clocked a track record of 51.46 to lead by a commanding fashion and Flock floundered, relinquishing her spot on the podium to spark jubilant celebrations among a sizeable British contingent at the Olympic Sliding Centre.
An emotional Yarnold said: "I'm overwhelmed and exhausted. I don't really know how it happened.
"After the first run I wasn't sure whether I was going to be able to finish the race because my chest infection was so bad I was struggling to breathe and I got here only with the help of my team.
"My physio Louise just gave me a talking to before run two (on Saturday) and reminded me that I can do it and we can do it together.
"I guess four years ago, three years ago the whole team all dared to dream that this was possible and I just went with all them (and) we managed it."
Britain have now equalled their record Winter Games medal tally of four - and there are plenty of further opportunities to come.
It is the first time ever Britain have won three Winter Olympic medals in the same day, overtaking the record two from Chamonix in 1924.
And Yarnold is now Britain's most decorated Winter Olympian.
Only figure skaters Jeannette Altwegg and pair Jayne Torvill and Christopher Dean, who have one gold medal and one bronze each, also have multiple medals for Britain. And now Yarnold is in an exclusive club all of her own.