Great Britain's women dominated Beijing silver medallists Argentina, but should have been more clinical in their 2-0 victory in the Olympic test event at the Riverbank Arena.
Coach Danny Kerry criticised the side for lacking "fire in their bellies" after their second-half performance tailed off.
"We can't just have 45-minute performances. The players have to have fire in their bellies and - apart from the backline which stood firm and Beth who made some good saves - I was looking for people to step up and show what kind of people they are," he said.
The hosts took the lead in the 15th minute when striker Alex Danson expertly turned her marker to fire home a trademark reverse-stick shot from the top of the circle.
Argentina's indiscipline cost them in the first half - they received four green cards and one yellow - and while they were temporarily down to nine players GB doubled their advantage.
Captain Kate Walsh slipped a penalty corner back to the post where Ashleigh Ball swept the ball into the roof of the net.
After Crista Cullen missed a 43rd-minute penalty stroke GB allowed Argentina back into the game and gifted them some unnecessary chances.
But despite a number of penalty corners they could not beat goalkeeper Beth Storry, who made a couple of good saves.
"I thought the first half was okay," said Kerry, whose side will face Argentina again in Sunday's final. "We attacked well and had lots of opportunities and there was a bit of spark about us."
He added: "I thought we started the second half brightly and missed a stroke but after that I was very disappointed indeed with the next 25 minutes."