Pippa Funnell's attempt to collect the Rolex Grand Slam reward of $250,000 (£160,000) started well here yesterday, when she held the lead on Primmore's Pride at the end of the first day of dressage.

Funnell, who won two other four-star events this year at Lexington and Badminton, leads Zara Phillips, the 22-year-old daughter of the Princess Royal on Toytown. A clutch of American riders, who are trained by Zara's father, Captain Mark Phillips, occupy the next three places, with Stephen Bradley, who won here in 1993, lying third on the Russian-bred From.

Leading up to the Burghley Horse Trials, Funnell had said that she had been "mentally shattered by indecision". Two of her top horses, Supreme Rock and Cornerman, had developed minor problems, forcing revised plans. She had intended to ride Supreme Rock and Primmore's Pride, leaving Cornerman for the defence of her European title at Punchestown in Ireland in two weeks' time.

Supreme Rock is now off for the rest of the year; Cornerman is being kept "ticking over" for the European event, but will not be risked if Funnell has any doubts about his well-being.

"The sport revolves around horses and their best interest. I couldn't risk Cornerman if he were only 99 per cent right," she said. So Walk on Star, her second mount here, will probably miss the cross-country and show jumping as he may be required for the Europeans.

Zara Phillips tackles her first four-star cross-country tomorrow. "The whole thing is big. I'll just have to keep thinking and keep riding," she said.