Hopes of an early British gold medal at the Rio Olympics ended in the gutter as cyclist Geraint Thomas crashed in the final stages of the men's road race.
Thomas had been in with a chance of a medal at Copacabana but slid off the road on the final descent of the route with 10km of the 237.5km left.
He picked himself up to finish 11th, one place ahead of team-mate Chris Froome, who won his third Tour de France last month.
But things looked more positive in the pool, with Adam Peaty smashing his own world record to take first spot in his heat of the 100m breaststroke, almost two seconds ahead of Scotland's Ross Murdoch.
Team GB's James Guy, Max Litchfield, Hannah Miley and Aimee Willmott also reached their respective 400m finals.
Britain's male gymnasts also shone as they qualified for the team final on Monday in third place, while the ladies' rugby sevens got off to a good start with comprehensive victories over Brazil and Japan.
There was also an astonishing performance by British equestrian star William Fox-Pitt.
The 47-year-old was placed in a coma just 10 months ago after suffering head trauma following a cross-country fall.
But after a stunning performance on day one of the Rio Games he leads at the halfway point of the dressage.
Rowers Katherine Grainger, going for a fifth Olympic medal, and her partner Vicky Thornley, also went through to the semi-finals of the women's doubles sculls, while Hannah Watson and Kyle Edmund both won their opening rounds in the tennis.
Andy Murray, who was beaming as he led Team GB's Olympic hopefuls into the Maracana for the opening ceremony on Friday, begins his campaign against Serbia's Viktor Troicki on Sunday.
But there was heartache and tears for judo player Ashley McKenzie, who was knocked out in the 60kg category.