World number one William Fox-Pitt has predicted a "big testing track" when the London 2012 eventing competition reaches its crucial cross-country phase on Monday.
The Great Britain team of Fox-Pitt, Mary King, Tina Cook, Zara Phillips and Nicola Wilson are in the bronze medal position after dressage behind favourites Germany and second-placed Australia.
Germany-based Japanese rider Yoshiaki Oiwa is the shock individual leader on a score of 38.10. Italian Stefano Brecciaroli is second and 56-year-old New Zealander Mark Todd, who is chasing his third Olympic title, lies one place behind.
But all that could change when riders tackle Sue Benson's 28-obstacle course over almost six kilometres of hilly and twisting Greenwich Park terrain.
"It is going to be a big testing track for the horses physically," Fox-Pitt said. "I don't think there is a particular jump that stands out, but on a course like this you just don't know.
"There could be a cumulative effect. Going very fast towards the end, mistakes could slip in, and I just hope we don't make any and that the horses stay sharp. When you are at an Olympic Games the atmosphere can affect certain horses in certain ways. It's very exciting that the marks are close enough together after dressage."
Cook, meanwhile, was faced with torrential rain, thunder and a threat of lightning during her dressage test in the main arena.
Sweden's Niklas Lindback - the next competitor after Cook - was delayed for 10 minutes by ground jury president Anne-Mette Binder as conditions deteriorated.
But New Zealander Andrew Nicholson, the world number two, blasted the weather hold as an "absolute disgrace" and claimed that decision had affected his preparations. Cook coped superbly with the elements, though, as her score of 42 penalties kept Britain firmly in the mix for medals.
"I have been eventing for 20-odd years and you do get this type of weather thrown at you," Cook said. "Unfortunately, we can't stop the competition and say 'I don't want to do it now'."