Charlotte Dujardin moved Britain into gold medal position in the dressage by setting a new Olympic record on board her horse Valegro.
The 26-year-old from Enfield scored 83.66%, which was more than four percentage points better than German Kristina Sprehe's previous best.
GB's average team score of 79.40% is 0.56% above Germany.
Dujardin and fellow riders Carl Hester and Laura Bechtolsheimer are aiming for Britain's first Olympic dressage medal.
The strong performance from Dujardin also put Britain in a strong position in the individual disciplines.
Her score was the best over two days of competition, with Dutch star Adelinde Cornelissen in second, Germany's Helen Langehanenberg third, Hester fifth and Bechtolsheimer seventh.
The team competition, where Dujardin holds the world record, resumes and concludes next Tuesday with the Grand Prix special test.
"It is an amazing opportunity to ride here and I really wanted to enjoy it," said Dujardin, who was only 0.42% short of equalling Edward Gal's world Grand Prix record with Totilas.
"I would hate to have gone in there, put too much pressure on myself and made mistakes. I just wanted to go in and do what I do normally and have fun. Having the crowd behind me like that was amazing."