Briton Laura Muir moved into the World Championship 1,500m final but fellow medal hope Katarina Johnson-Thompson suffered a heptathlon setback on Saturday.
Scot Muir, Sir Mo Farah apart, is arguably Great Britain's brightest podium prospect this week and she tracked Olympic champion Faith Kipyegon to cross the line in second place in four minutes 3.64 seconds.
Behind her were South Africa's Caster Semenya, dominant over 800m but still something of an unknown quantity over the longer distance, and world-record holder Genzebe Dibaba, evidence of how loaded the race was.
Both qualified, Semenya comfortably in third, but Dibaba had to rely on a fastest-loser spot after trailing home sixth in 4mins 5.33secs.
Muir, seventh at last year's Olympics in Rio, will return for the final at the London Stadium on Monday night and said: "I'm really happy. That was a really tough semi-final, so I'm just glad to have got through it.
"That was really strong, it could easily have been a final.
"I was boxed in a bit, but I knew not to panic. I was running on the inside, not wide. I think I've conserved as much as I can.
"I kept glancing up at the screen to make sure I didn't have too big a gap. I didn't want to waste too much energy.
"But the likes of Semenya and Dibaba were behind me and they are very strong sprinters, so I thought I wanted to keep out of harm's way and have a bit up my sleeve in case people came rolling past."
In the end Muir crossed the line 0.16secs ahead of South African Semenya.
Kipyegon said: "I know Muir is there, Semenya is there, so everybody can win the race.
"The cheering for Muir also gives me some motivation to do what I have to do."
Muir's team-mate Laura Weightman joined her in the final after finishing fourth in the second semi-final in 4:05.63.
While Muir's prospects looked bright, Katarina Johnson-Thompson's were dealt a blow by an awful high jump in the opening morning of the heptathlon.
The devastated 24-year-old collapsed to the mat, head in hands, after a third failure at 1.86m. It left her with a best clearance of just 1.80m from arguably her strongest event, a full 18 centimetres off her personal best and a difference of 233 points.
She returned in the evening to throw 12.47m in the shot put, her weakest event, and, more significantly, then clock a mightily-impressive 22.86secs in the 200m.
That run took her from 13th place to fourth, her total of 3,838 points 67 off third-placed Yorgelis Rodriguez. She finished the day 7/4 with Sky Bet to win a medal - she had been odds-on at the start.
It was a fine way to finish a tough day, which had started well with a 13.33 clocking in the 100m hurdles, the second fastest time of her career. But there remains ground to make up.
Germany's Carolin Schafer led on 4,036, ahead of Olympic champion and gold medal favourite Nafissatou Thiam of Belgium on 4,014.
Johnson-Thompson was a star-struck 19-year-old at the 2012 Olympics but, after being dubbed Jessica Ennis-Hill's natural heir for so long, is now expected to deliver medals on the big stage.
But Saturday's high jump failure left the Liverpool athlete with an awful sense of deja vu, having seen her medal hopes at the 2015 World Championships in Beijing ended by three no-jumps in the long jump and her Olympic dreams last year crushed by poor throws, a sixth-placed finish reducing her to tears.
Since returning from Brazil she has made drastic changes, splitting with her long-term coach Mike Holmes and leaving her home city of Liverpool to switch to a training base in Montpellier.
Meanwhile, Ethiopia's Almaz Ayana claimed a crushing 10,000 metres victory by a staggering 46 seconds in her first race in 11 months.
The Olympic champion, who set the world record of 29 minutes 17.45 seconds in taking gold in Rio, clocked 30mins 16.32secs to land another global crown.
Ayana's second 5,000m was astonishing, her time for the second half of the race of 14m 24.94s was quicker than her own championship 5,000m record.
Last summer the 25-year-old declared himself "crystal clear" when forced to answer doping doubters after obliterating the 23-year-old world record in Rio.
"My doping is my training, my doping is Jesus. Otherwise, nothing," she said then.
On Saturday's run, she said: "I am very happy to win this title, much more than when I won the Olympic gold because I have been sick this year and didn't expect it."
In the field, the men's long jump title went to South Africa's Luvo Manyonga, while Andrius Gudzius, of Lithuania, took gold in the discus.
