Every day for four years, Sanya Richards-Ross had to live with those 49.93 seconds. And every day for four years, Richards-Ross knew that she would have even less time than that to redeem herself.

The bitter taste of bronze from the 2008 Beijing Games fueled the 27-year-old sprinter, and when the starting gun fired Sunday night in Olympic Stadium, it was time to erase the bad memories. Four years after fading down the stretch, Richards-Ross tore around the curve and charged toward the finish line, winning gold in the women’s 400 meters in 49.55 seconds.

“I worked so hard for that, and I prepared for this moment over and over for the last four years,” Richards-Ross said.

Her American teammate DeeDee Trotter took bronze, finishing in 49.72, just two-hundredths of a second behind Britain’s silver medallist Christine Ohuruogu.

In addition to her bronze from the 2008 Games, Richards-Ross had won two Olympic golds in the 4x400 relay, one in Athens and one in Beijing. But Sunday marked her first individual gold medal.

“If anyone can get it, it was her,” Trotter said of Richards-Ross. “She has worked hard for it, and she has completed her dream for herself.”

She’ll race in the women’s 200-meter event and the 4x400 relay later this week.

The victory was the United States’ first track and field gold medal at these Olympics, but the Americans have more in their sights.

Matthew Centrowitz (Broadneck High) might face an uphill battle in the men’s 1,500 race, but he posted his best time of the year to advance into Tuesday’s final.

“I am healthy, I am confident. Those two mixed together could be good,” Centrowitz said of his chances in the final.

Running in the night’s second — and faster — heat, the 22-year-old Centrowitz crossed the finish line fifth in 3:34.90, just 0.28 of a second ahead of the sixth-place finisher, Turkey’s Ilham Tanui Ozbilen, who did not advance.

American Leo Manzano also advanced to the Tuesday’s final, finishing the first heat in fourth place with a time of 3:42.94. American Andrew Wheating failed to advance, finishing the first heat in ninth place with a time of 3:44.88. Morocco’s Abdalaati Iguider posted the fastest semifinal time, winning the second heat with a time of 3:33.99.

While the Americans took two medals in the women’s 400, they won’t fare nearly as well in the men’s. With LaShawn Merritt sidelined because of a hamstring injury, the United States will have no chance to defend its gold medal in Monday’s men’s 400 meter final.

One day after Merritt, who won the event at the Beijing Games, pulled up with a hamstring injury in a first-round heat, his two American teammates failed to advance past the semifinal round.

Tony McQuay finished his semifinal race in fourth place with a time of 45.31. And Bryshon Nellum finished his race in third place with a time of 45.02 seconds, just 0.17 of a second away from qualifying for the final.

Lalonde Gordon, of Trinidad and Tobago, posted the fastest semifinal time, finishing his race Sunday night in 44.58 seconds, a personal best.

In his semifinal race, Oscar Pistorius, the first track athlete to run on prosthetics in an Olympic Games, finished in last place and did not advance to Monday’s final.

Pistorius said his goal in London was to run well enough in the first-round heats to merely qualify for the semifinals, which he did successfully Saturday, finishing second in his race and crossing the finish line in 45.44 seconds. Lined up in a more competitive eight-man field one day later, the South African finished in 46.54 seconds Sunday night.

“The whole experience is mind-blowing. . . . It’s a dream come true,” he said.

His Olympics are not over. Pistorius is expected to compete in the 4×400 relay later in the week.