The French captain's two miracle threes kept France unbeaten atop Group B at 4-0 and Britain winless after wrenching losses to Australia, Canada and Russia that had the Olympic Basketball Arena fans roaring but deflated in the end.
Dumerc, who finished the game with 14 points, was well aware that she was not the most popular athlete in London following her sharpshooting display.
"I think it is the opposite. I think everyone hates me," the 30-year-old point guard said as a wide grin covering her face.
Britain was ahead 67-64 when Dumerc let fly an off-balance, left-handed three-point heave that swished through the net with six seconds to play and silenced a roaring home crowd.
Undaunted, the plucky British team led by tenacious captain Johannah Leedham got right back to business in the five-minute overtime. Leedham scored her team's first seven points in the extra time as Britain built a 76-72 lead.
But with the game tied 77-77 and only seconds remaining, Dumerc went to work again.
This time she stepped away from the three-point line, moved to the side and launched another guided missile through the hoop with .2 seconds left before the final buzzer.
Dumerc said this sort of rescue mission was a first for her.
"Never twice in a game," said Dumerc, who scored 14 points in the game. "But on this kind of moment you don't think. You just throw the ball. For me, we lost. We already lost.
"You don't have to think. You just have to play and this kind of moment is magic. You will ask me to do it again and I will miss."
The 24-year-old Leedham scored 29 points for Britain.
"Everybody played with their heart on their sleeve. That's the ethos of our team. That's what we do," she said.
"To compete with the teams and be so close and yet so far, it's disheartening but we've come a very, very long way and we're so close. You know we're going for that win against Brazil and that would be huge for us to get that first Olympic win."
The Czech Republic made a perfect start but the US enjoyed the happier ending as they extended their Olympic winning streak to 37 games with an 88-61 victory in.
The Czechs scored the first 10 points of the game and continued to rain down three-pointers for the rest of the first quarter before the US ratcheted up their defensive pressure to take control of the contest.
"We started 10-0, which was a perfect start for us," Czech forward Michaela Zrustova said. "For the first quarter, we were happy. For the first half, we felt we were able to keep pace with the US team."
Tereza Peckova's long-range basket at the end of the first quarter gave the Czech Republic a 26-24 lead, built on 6-of-10 shooting from beyond the arc.
After storming ahead in the second quarter, a 10-2 run after halftime put the US 58-40 in front and their stingy defence held the Czechs to just nine points in the third as the Americans (4-0) turned the game into a rout.
Diana Taurasi led the US attack with 18 points and Tina Charles scored 16. Zrustova had 15 points for Czechs, who dropped to 1-3.
Maya Moore added 14 points and was a catalyst in the US team's defensive turnaround.
"That's my job. When I get into the game, I want to create chaos for the other team as well as I can. I feel like I've got some athletic ability and some long arms and quick hands," she said.
"I try to get in the passing lane, get a tip, get a steal and hopefully create some momentum for our team."
US coach Geno Auriemma, who coached Moore at the University of Connecticut, said that was exactly why she was picked to be on the team.
"Maya Moore is just a high energy player. She plays every possession to the end. Not everybody does that," he said.
"For Maya, every defensive possession is an opportunity to get a steal. Every shot is an opportunity to get a rebound. It's just her make-up. It's who she is. She works incredibly hard on the floor, as hard as any player I've ever seen at any level."
Auriemma's halftime teamtalk focused on stepping up on defense.
"I think our defensive intensity picked up," Tamika Catchings said.
"I think our third quarter was probably one of the best quarters we've had since we've been playing together. Now we just got to start off the game the same way."
The US will finish preliminary round action against China on Sunday before they line up for a quarter-final two days later in the pursuit of a fifth straight Olympic title.