The London 2012 eventing cross-country course took a toll at Greenwich Park today - 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.

An elated Wilson, who was called into the Olympic team after Piggy French's horse DHI Topper W suffered an injury last month, remained on her dressage score of 51.70 penalties.

Fans ran over to congratulate her after she finished finishing the course, some waving union flags. She pumped her hand in the air and waved.

"He was just unreal, what a fantastic horse," Wilson said.

"This is his favourite stage by far. When he gets to go cross-country day it's like all his birthdays and Christmases have come at once.

"He never fails to give me the wow factor."

Australian Christopher Burton commanded the initial headlines, going clear inside the time by a remarkable 17 seconds.

Burton, who won the World Equestrian Festival eventing title at Aachen in Germany earlier this month breezed around the 5,728-metre course on HP Leilani.

And it strengthened Australia's position in the team competition after they held the silver medal place overnight.

But 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.

King danced with happiness when she looked up at the scoreboard and saw she was in the lead.

She ran over to kiss her daughter before speaking to journalists.

"It was a great place to be stopped, so it was an advantage to us really," King said.

"You can pick up the speed before they stop the stop watch that they've had running through your stoppage time.

"There was a nice straightforward fence ahead and I managed to gallop through it at a really nice stride."

The third British team member, Zara Phillips, attacked the course with confidence from the start on her Olympic debut.

The Queen's granddaughter, watched from the main arena stands by the Duke and Duchess of Cambridge, Prince Harry and the Duchess of Cornwall, did not disappoint her audience.

Despite her horse High Kingdom's relative inexperience at the top level, they relished the challenge.

And they became the fourth combination to go clear inside the time, after Burton, Wilson and Germany's Ingrid Klimke.

Phillips punched the air as she went through the finish to an ecstatic reception from an adoring crowd as she finished on her dressage score of 46.10 and became the fourth clear round.

She told BBC2: "He (High Kingdom) was awesome.

"He is very suited to this course, but he lost a front shoe, so he really stepped up today."