Great Britain's Gemma Gibbons charged into the final of the women's under-78kg division after stunning world champion Audrey Tcheumeo of France at ExCeL.
The North Arena 2 crowd was again in full voice as they sensed Britain's first judo Olympic medal since 2000.
Gibbons, 25, only stepped up a weight division this year.
However, the University of East London Masters student thrilled the home crowd as she battled past Yahima Ramirez of Portugal, Mongolia's Lkhamdegd Purevjargal, the seventh seed, and then beat Dutchwoman Marhinde Verkerk in her quarter-final.
Tcheumeo beat Gibbons in the Romania World Cup event in June, and was soon on the offensive.
However, Gibbons held her own, defending the attacks and looking to counter as the bout headed into the final 90 seconds without score.
Roared on by the home support, the British fighter went forwards, but the contest moved into three minutes of sudden-death judo.
Gibbons, recovered from a shoulder injury which needed surgery, began to tire, but her opponent also looked a spent force - and the Briton produced a stunning ippon to claim a place in the final where she will fight for gold against American Kayla Harrison, who disposed of top-seeded Brazilian Mayra Aguiar.
Tcheumeo must pick herself up for a clash against Abigel Joo of Hungary for bronze.