Sophie Hitchon broke her own British record in the hammer with a throw of 71.98m to reach Friday's final.
The 21-year-old from Burnley struggled with her first two efforts but beat her previous best by 37cm with her final throw to finish fifth in her group and qualify in 10th place.
It was the fourth time Hitchon has beaten the British mark this season.
"I knew it was there but you only have one throw left, so it was all or nothing," Hitchon told BBC Sport.
"The first one clipped the cage and went 67m, then the next one felt good. It just went out of the sector."
Poland's European champion Anita Wlodarczyk threw furthest in qualifying with an effort of 75.68m.
Hitchon, who is a former ballet dancer, said she fed off the capacity 80,000 crowd at the Olympic Stadium.
Before her qualification was announced, said she would be content whether she made the final or not.
She added: "It's a morning session, it's only qualification and look at the crowd. If I don't make it, I'm not really bothered. I threw my personal best and there's nothing more I can do."