In a close race, Athing Mu comes away with an expected win.

For the first time in her professional career, the American runner Athing Mu looked as if she could lose an 800-meter race.

Mu typically dominates her races with a unique and effortless stride that features little arm movement and, seemingly, little energy. While her world-class competitors are grunting and grimacing, trying to keep pace, Mu is known to keep a stoic face and move gracefully as she builds a gap between herself and the field.

But that’s not what happened on Sunday night in the 800-meter final. Typically, Mu turns to another gear with 200 meters left in the race, leaving her competitors behind. But this evening Keely Hodgkinson of Britain kept pace.

The crowd at Hayward Field in Eugene, Ore., roared as Mu and Hodgkinson ran side by side for the final 100 meters. Mu outleaned Hodgkinson at the finish line to win with a time of 1 minute 56.30 seconds, beating Hodgkinson by just eight-hundredths of a second. Mary Moraa of Kenya finished third. Hodgkinson also finished behind Mu at the 800-meter Olympic final last summer, setting a national record with a time of 1:55.88.

Hodgkinson and Mu each hold national records, have won Olympic medals and, now, world championship medals, too. They are the same age — 20 years old — and each other’s fiercest competitor in the event.

Hodgkinson said that she was happy with another silver, this one a world championship medal, but that it was somewhat bittersweet to have been “so close” to gold.

Mu said that she had been expecting Hodgkinson to push her through the finish line, and that she had felt, but not seen, Hodgkinson on her left side during the final 100 meters.

“I was just happy it was over,” Mu said. She said that Sunday was a “rough day” and that she did not feel as though she was in her peak physical shape. “Thankfully I got the gold, and thankfully I still had something in me so I could run through the line and finish strong,” she said.

It was one of the closest races she had ever run. In June, she faced a similarly tight race at the U.S. national championships when Ajeè Wilson unexpectedly challenged Mu just ahead of the finish line. Still, Mu said she didn’t feel as if she ran harder on Sunday than she would have in any other race.

“Most of the races I run, they aren’t really tight races with other people,” Mu said. “But I think this is just how any race would be if I run with someone else that’s really competitive.”

Mu is the first American woman to win the 800-meter world championship. She joins Donavan Brazier as one of two Americans to win the 800-meter world title.