Steph Houghton's third goal in as many games was enough to maintain Team GB's flawless Group E campaign at London 2012 on a record-breaking night at Wembley.
Such was the magnitude of the result, Powell's players and the 70,584 crowd - by far the largest ever for a women's game in Britain - celebrated at the final whistle almost as if a gold medal had been won.
But their manager was as stony-faced as ever and urged caution ahead of Friday's quarter-final against Canada in Coventry.
"Every game, we've done well, we've won the game, we've topped the group," said Powell, whose side have yet to concede a goal.
"The danger is that everybody starts jumping on this 'medal'.
"We still have to play Canada. They want a medal as much as anybody else.
"We're not in a position yet to get a medal."
Powell was similarly ambivalent about her side's performance on Tuesday night, despite Britain creating the better of the chances and Kelly Smith missing a penalty.
She said: "Yes, we played well in patches. It wasn't the complete performance by any stretch of the imagination.
"But we did a job, very resilient, worked hard.
"You saw the girls at the end. They were dead on their feet."
Powell was nevertheless "really pleased" with the outcome and the "electric" support of the unprecedented crowd.
"I must say the crowd were really inspiring tonight," she said.
"I think they gave us a little bit of an edge, which was great for us."
She added: "As a manager, certainly I've never experienced it.
"When I talk on the sidelines, the girls can usually hear me.
"Tonight was a good excuse for them to ignore me, which is what they tried to do...
"We actually spoke about it beforehand that 90% of the crowd were there to support Team GB and, 'Go out there and enjoy it and they'll get behind you'."
Powell did allow herself to heap praise on Houghton, who is emerging as one of Team GB's stars of the entire Olympics.
"I don't know if she's in line for the golden boot," Powell said.
"She's absolutely loving every minute of it.
"So, for her, all the hard work has paid off."
Arsenal star Houghton said: "It's unbelievable and amazing. To play in front of more than 70,000 people, you couldn't dream of a better day.
"There was no pressure on us because we knew we had qualified for the quarter-finals already.
"But now we are looking forward to Friday against Canada and we will just try to recover and then focus on that match."
Powell had been hoping Tuesday evening's historic match would help create a legacy for women's football beyond the Olympics, and she said: "I think tonight partly achieved some of that."
She added: "This is a wonderful experience that, probably, we might not ever experience again.
"Part of the process is fun and enjoyment. Have a good time. These will be fantastic memories. We've got to embrace it.
"It's a great opportunity to showcase women's football.
"We're just trying to have a good time and I think it makes it easier when you win.
"It elevates everybody, even though we're knackered."
Brazil boss Jorge Barcellos refused to use tiredness as an excuse for his side's defeat after they were left infuriated by their late departure from Cardiff to London.
The bus designated to transport them last night broke down and it took five hours to replace it, leading to allegations of a plot.
"That has nothing to do with us losing," said Barcellos, who also insisted Britain had to improve if they wanted to win a medal.
But he added: "If GB has to get better, we have to get much, much better."
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