Great Britain's Lisa Dobriskey and Laura Weightman qualified for the final of the women's 1500m, but Hannah England missed out.
Dobriskey, 28, eased home in 4 minutes 5.35 seconds to ensure she was one of the five who qualified from the first semi-final.
"If ever there was a lesson to anyone to keep going to the line... Laura Weightman did fantastically well. Two women in the 1500m final is fantastic. Lisa Dobriskey was cruising down that home stretch. She has a great chance."
Weightman, 21, qualified as one of the fastest losers to reach the final in her first ever Olympics.
England, who has struggled with injury this year, was ninth in her heat.
Dobriskey's place in the final was never in doubt but the former world silver medallist admitted: "I was so nervous about this race. I just wanted to make an Olympic final.
"It was a bit of a scrap at the end but that seems to have been the case in a lot of the qualifying rounds and I am just happy to be through."
Weightman, who is coached by Olympic 1500m silver medallist and BBC athletics commentator Steve Cram, dipped at the line to finish just ahead of Lucia Klocova with a personal best time of 4:02.99.
Weightman delight at making 1500m final
"It feels absolutely unbelievable. Personal best, Olympic final. What more can I say," said the Northumberland runner.
The first semi-final was won by Turkey's Asli Cakir Alptekin in 4:05.11, while Russia's Ekaterina Kostetskaya and Americans Morgan Uceny and Shannon Rowbury also qualified for Friday's final at 20:55 BST.
In the second semi-final, Ethiopian Abeba Aregawi crossed the line first, followed by Turkey's Gamze Bulut, Russian Tatyana Tomashova, Bahrain's Maryam Yusuf Jamal and Kenya's Hellen Onsando Obiri. Natallia Kareiva of Belarus joined Weightman as one of two fastest losers.