Germany's Ulla Salzgeber held on to a narrow lead as she moved into the third and final stage of the individual dressage on Monday morning.
Salzgeber on Rusty scored 74.840 in the second stage, the Grand Prix Special, to stay top of the standings on 76.524.
Anky Van Grunsven of Holland, who leapfrogged Spain's Beatriz Ferrer-Salat into second, is 0.520 behind.
Britain's Carl Hester came ninth as the top 15 went throught to Wednesday's final stage, the Grand Prix Freestyle.
But there was disappointment for compatriot Richard Davison, whose missed out on qualification after finishing 22nd.
Hester, 37, with his horse Escapado followed up on his outstanding team
display, with another nerveless test.
His score of 71.553% saw him occupy fourth place for much of the
morning until some fine performances late on.
"I am absolutely thrilled with that," said a delighted Hester.
"In a competition six weeks ago, I was last and now I am in the Olympic final.
"He (Escapado) is hugely talented, just a fantastic horse.
"His only problem is that he is not very experienced and that is where the errors come.
"That was where we had difficulty but he is so talented that he able to
overcome that. There is nothing that he cannot do."
Salzgeber, who helped Germany clinch their sixth successive team dressage gold medal at the weekend, went into the second stage leading by 4.00.
But the German saw that margin slashed when she was unable to match her brilliant opening ride and and van Grunsven on Salinero scored 77.800 in the special.
The top 15 now progress to Wednesday's freestyle round, where competitors perform their own routine to music.