Pearson posted the highest score in Grade Ib of the team event - 74.682 per cent on Gentleman - but it was not a vintage display by the nine-time Paralympic champion in wet and windy conditions.
Gentleman struggled for fluency in parts of the test, while Pearson picked up two penalty marks for an error.
'That was totally my fault,' he said. 'I've been reading the test for about a week now. I read it last night before I went to the opening ceremony, and then again this morning.
'It was just two circles, and I got them the wrong way around. It was rider error.
'It wasn't our best test in the world, but I had warned everyone about Gentleman's sense of humour. He warmed up perfectly, and then he came into the arena and realised a few thousand people were watching him.
'He lost all of his energy, and I had to nurse him throughout the arena. It's just in his head, really. He can be a bit cheeky sometimes when he goes in the arena.'
Austrian Pepo Puch is considered the rider most capable of ending Pearson's run in the individual competitions, which get under way on Saturday, and he finished just behind his rival on 73.636 per cent - ahead of Joann Formosa of Australia on Worldwide PB (71.955 per cent).
Earlier, Natasha Baker proved the star of the show in a Grade II field of 23 riders, as an impressive score of 76.095 per cent aboard Cabral saw the British rider top the standings.
No rider seriously threatened Baker's score, and she finished top after all 23 combinations had completed their tests.
Germany's Britta Napel went closest, posting 72.571 per cent on Aquilina 3, while Canadian Lauren Barwick scored 72.095 per cent with Off to Paris.
Team medals will be decided on Sunday after scores are combined with individual competition results across all five grades.