Lamaze, from Schomberg, Ont., was one of five riders to successfully negotiate the first-round track, featuring jumps set at a height of 1.6 metres.
In the jump-off, Lamaze guided Powerplay to another clear round with a time of 44.55 seconds. It was not quite fast enough to catch Penelope Leprevost of France, who was timed in 43.77 seconds riding Dame Blanche van Arenberg.
"Powerplay is going to learn to go a bit faster, but he's not much of a race horse," said Lamaze of the nine-year-old Holsteiner gelding acquired three months ago by owner Artisan Farms.
"He spends a bit of time in the air and he has a big stride, but he's learned a lot. Penelope didn't beat me by that much, though, so he's getting better at it."
Vancouver's Tiffany Foster, who is coached by Lamaze, finished eighth with Verdi III after incurring one time fault in the opening round.