Nina Derwael seizes gold in the uneven bars as Sunisa Lee falters.

Nina Derwael of Belgium took advantage of her rivals’ mistakes and nerves during the uneven bars final on Sunday to win the gold medal in her specialty.

Derwael, a two-time uneven bars world champion, did not perform as well on bars as she had in qualifying last week, when she finished first, but her routine was good enough — and difficult enough — for her to win. Looking as if she were gliding in the wind, she soared above, under and between the bars and was given 15.2 points for her performance. After the announcer called her the winner, Derwael wrapped herself in the Belgian flag and flashed a huge smile toward her small cheering section in the mostly empty Ariake Gymnastics Center.

Anastasiia Iliankova of Russia was second on the uneven bars, with 14.833 points, to win the silver medal. Sunisa Lee of the United States won the bronze, with 14.5 points.

The Belgian winner was supposed to vie for the gold with Lee, who also is an uneven bars specialist. Lee usually has the hardest uneven bars routine in the world and is rewarded for it with a high score.

On Sunday, though, three days after she won the gold medal in the all-around, Lee performed a shaky routine and failed to connect several of her skills, which lowered her difficulty score. Her execution wasn’t as sharp as usual, either. Once she finished her routine, she began shaking her head with disappointment.

The day before the event, Lee admitted that she had felt pressure to win gold on the bars, particularly after winning the all-around title.