The London 2012 eventing cross-country course took a toll at Greenwich Park - but did not prevent Great Britain from strengthening its gold medal bid.
Superb cross-country rides by Nicola Wilson and Mary King turned up the heat on Germany and Australia, countries that were ahead of Britain overnight.
But seven of the first 27 combinations fell on a course that appeared to be very slippery in places and challenging in terms of its fences.
Canadian rider Hawley Bennett-Awad required medical attention after falling at fence three, with her condition described by organisers as "stable" while she underwent further assessment in an on-course ambulance.
Belgian Carl Bouckaert fell at the same fence, while Japan's Takayuki Yumira also parted company with his mount Latina, which required the horse being treated.
The combined effect meant that King and Imperial Cavalier were about 25 minutes late starting their round.
Wilson and Opposition Buzz gave the host nation a flying start. The combination is regarded among the world's leading cross-country pairings, and they lived up to that billing in front of an estimated 50,000 crowd.
The world and European team gold medallists delivered a brilliant round, finishing almost 12 seconds inside the time allowed of 10 minutes, three seconds.
Australian Christopher Burton commanded the initial headlines, going clear inside the time by a remarkable 17 seconds.
Despite the delay King, utilising all her experience, kept concentration, and she completed the course with just 1.2 time penalties, taking her overall score to 42.10 and into the lead ahead of Burton despite being briefly held on course while Yumira's horse was attended to.