Doyle ran well in her semi-final, nailing her stride pattern, and seemed to have sewn up an automatic qualifying position in the second of three semi-finals. However, Ashley Spencer of the USA made an incredible recovery from clipping a hurdle on the home straight with a raw speed finish that pushed the Briton down into third.
Her time of 54.99 proved enough, though, to progress into the final which takes place in the early hours of Friday morning.
Dalilah Muhammad of the United States qualified fastest in 53.89 and looks the woman to beat, although Spencer must also be a contender if she can avoid hitting barriers with what is a high-risk technique. Double world champion Zuzan Hejnova of the Czech Republic won the first semi-final.
It looks like a wide open final, which Doyle will happy to be a part of after failing to reach that stage at London 2012.
Doyle said: “It wasn’t the best way to get into the final but at least I’ve got there. I stepped off the track knowing I’d given everything there. I actually executed the race really well. It was a really good stride pattern, nice and smooth.
“I think the only thing was I tired up a little bit towards the end as I was fighting for that line. I was quite tired coming back in off [the heats] as well. I think you saw that in all the times. Nobody ran what they were running this year.
“I’ve made it now. I can go back. I’ve got a day off. I can relax, recover and get ready for the final. Malcolm [Arnold, coach] will be happy with that run. A lot happier than he was after the heats anyway. I think I can go into the final confident now.
“It’s the best feeling ever to have made the Olympic final. I’m just delighted. I knew it wouldn’t be easy. People obviously see you as ranked sixth in the world but it doesn’t really mean anything when you get to the Olympics because everybody raises their game.
“So I’m just glad I’ve put myself in the position where I can now go and challenge in the final. It’s completely open – nobody is dominating the event so I think it’s just about who gets it right on the day.”
Dina Asher-Smith squeezed into the 200m final as the fastest loser after finishing fourth in 22.49.
In a stacked semi-final also featuring world champion Dafne Schippers and newly-crowned Olympic 100m champion Elaine Thompson, the 20-year-old went off hard from lane seven and paid for it in the final stages, tying up as Schippers won in 21.96.
Jodie Williams went out as she finished eighth in her semi-final in 22.99.
In the long jump Lorraine Ugen with 6.65m and Jazmin Sawyers with 6.53m both qualified for the final, but world silver medallist Shara Proctor could only manage a best of 6.36m and went out.
Robbie Grabarz beat the height that secured his London 2012 bronze but just missed out on the high jump podium in Brazil.
Four years on from having the home crowd on their feet, a sparsely-filled Olympic Stadium witnessed the 28-year-old’s attempt to repeat the feat in Rio.
Grabarz immediately cleared 2.20m and then 2.25m at the second time of asking, before soaring over 2.29m - the height that secured him bronze in London.
The Briton appeared to have then managed 2.33m, pumping his fists in celebration only to see the bar belatedly fall and the white flag replaced by a red one.
A perplexed Grabarz appealed the decision and the judges eventually relented, reinstating the jump that equalled his season’s best.
However, 2.36m proved too high for the 2012 European champion, seeing him finish joint fourth with Ukraine’s Andriy Protsenko.
Canada’s Derek Drouin won gold with a season’s best-equalling 2.38m, with Mutaz Essa Barshim of Qatar following home in silver after posting 2.36m. Ukraine’s Bohdan Bondarenko won bronze with an effort of 2.33m.