Olympic 800m champion Keely Hodgkinson was beaten for the second consecutive Diamond League race as she finished behind Lilian Odira in Eugene.
The Kenyan athlete, who is the reigning world champion over the distance, surged past the Briton in the home straight to win in a time of one minute and 56.19 seconds.
It was a season’s best performance from Odira, with Hodgkinson crossing the line second in one minute and 56.73 seconds, well short of the British record she set in Stockholm in June, when Audrey Werro triumphed.
It was also way off the oldest world record on the track of one minute 53.28 seconds set by Czech athlete Jarmila Kratochvilova in 1983.
However, that never looked like being a realistic goal in Oregon, with Hodgkinson emerging for the race with both knees heavily strapped – after a bad fall – and as a consequence, making a laboured start.
It has been a mixed season for Hodgkinson, who had to pull out of the 400m final at the UK Athletics Championships last month after feeling “a little twinge”.
The 24-year-old has been competing over the shorter distance in a bid to improve her first-lap speed and challenge for the 800m world record this summer.
Meanwhile, Britain’s Georgia Hunter Bell lowered her personal best in the women’s mile with a time of four minutes 18.52 seconds but that time was only enough for eighth.
Laura Muir faded in the closing stages to end up a distant 10th in a race won by American Nikki Hiltz, who timed her sprint to perfection to hold off Kenya’s Dorcus Ewoi and world record holder Faith Kipyegon.
World champion Melissa Jefferson-Wooden won the women’s 100m in 10.78 seconds with Sha’Carri Richardson second in 10.79 and Britain’s Dina Asher-Smith crossing in ninth.
Teenage American sprinter Tate Taylor, 19, marked his Diamond League debut with a personal best of 19.75 seconds as he won the men’s 200m ahead of Olympic gold medallist Letsile Tebogo.
In the men’s 100m, Nigeria’s Kayinsola Ajayi beat world champion Oblique Seville with a time of 9.84 seconds, with American Christian Coleman in third.