Simpson's opponents tomorrow include Ethiopia's world indoor 3,000m champion Genzebe Dibaba, Sifan Hassan of The Netherlands, the European 1500m champion, as well as Sweden's adopted  Ethiopian Abeba Aregawi, world 1500m champion and European silver medallist, and adopted Eritrean, Meraf Bahta, the European 5,000m champion.

"It is a uniquely exciting time for women's 1500m running," said the American 2011 world champion, whose best of 3min 57.22sec has her second in the world lists this year behind Hassan's 3:57.00.

"Not just in terms of talent, but the depth of the talent.

"Nobody knows who will win.

"There is no strong, heavy favourite."

Swedish fans at the picturesque stadium which hosted the 1912 Olympics will have high hopes of victory in one of the most highly competitive events of the meeting.

Bahta, who held off Hassan to win her European gold in Zurich last week, commented: "My pb (personal best) for 1500 is 4:03, and I want to run under four minutes in Stockholm, or even a 3:59 or 3.58."

Aregawi said: "I haven't had such good running recently so I hope I am going to be better next race.

"I am happy for the other Swedish runners like Mera and Charlotta Foukbourg.

"I had the good time, now they are coming up."

Hassan said she had been training with Bahta in St Moritz and Flagstaff, Arizona.

"It's good to train together because we can push each other and help each other," she said.

"Mera is a very strong athlete.

"She never gives up."

Sifan hassan beats Abeba Aregawi to European 1500m gold in Zurich ©AFP/Getty Images

The women's long jump is likely to be equally competitive, featuring as it does the world and Olympic champion Brittney Reese and her US compatriot Tianna Bartoletta, the 2005 world champion who has returned to the field event after a seven-year diversion to sprinting following a serious knee injury.

Reese, despite having jumped 7.25m last season, has re-vamped her technique this year.

"Even though I jumped 7.25 last year I felt I wasn't where I need to be on the runway,"Reese said.

"Everyone  knows I'm the Queen of Fouling!

"It's something I'm trying to change, and as I have no major championships this year I'm trying to experiment.

"I used to just run, and use my speed.

"Now I do a four step walk which helps me get speed up quicker.

"And on the take-off board I am trying to limit my natural dip, and to be taller."

Bartoletta, who has the four top long jumps this year with a best of 7.02m, explained that her involvement with the US bobsled team, for whom she raced in the World Cup in 2012, had played a key part in her return to the event this year.

"I got really fast, so when I got back to runway after seven years away I was way too fast," she said. "It was scary. Because the faster you run on the runway faster that last step has to be.

"But the reason I came back was I did a short stint on the US bobsleigh team as brakewoman, and I realised pushing  a bobsled on an ice track running just as fast on the runway while launching myself into the back of an accelerating bobsled - this is way more frightening than taking the penultimate step of a long jump!"

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