Sunisa Lee wins the all-around gold medal.

Sunisa Lee, an American gymnast who spent a lifetime aspiring to finish second to Simone Biles in the all-around because that was the best anyone could do, exceeded those expectations on Thursday night, winning the Olympic gold medal at the Tokyo Games.

With Biles out of the event after withdrawing from it because of mental health concerns, Lee took advantage of the opening left behind by the gymnast considered to be the best of all time. Lee, who is 18 and from St. Paul, Minn., hit routine after routine, often as if she were at practice, not at the most important competition of her life.

She won the gold in the all-around, which determines the best overall gymnast, just two days after rallying her teammates to win silver in the team final. Biles had pulled out of the team event after competing on the vault, the first of four events. On the next event, the uneven bars, Lee decided to perform her hardest bars routine — which is the most difficult in the world — instead of the easier one she had planned because she knew the team would need every tenth of a point to win a medal. Her 15.4 points on bars was the highest score of the night.

Later, Lee competed on the floor exercise after not practicing her floor routine for two days. She wasn’t initially selected to perform it at team trials because each country chooses only three of its four gymnasts to compete on each apparatus. With Biles out, though, she had no choice.

In the all-around, she brought the same resolve to make it to the podium. And in the end, she made it to the top of it.