Kosgei broke the mark of 2:15:25 set by Radcliffe in the London Marathon on April 13, 2003 as she won in Chicago for the second straight year.
Kosgei, the 25-year-old who won the London Marathon in April and clocked the fastest half-marathon in history this year of 1:04:28 at the Great north Run, quickly separated herself from the women’s field as she ran with two male pace-setters.
She crossed the finish line alone, with Ethiopians Ababel Yeshaneh and Gelete Burka a distant second and third in 2:20:51 and 2:20:55.
While the IAAF recognizes the 2:17:01 clocked by Mary Jepkosgei Keitany at the 2017 London Marathon as a “women only” world record posted without male pace-setters, it’s Radcliffe’s mark — so long untouchable — that has been the grail for female marathon runners.
Kosgei’s performance continued a remarkable weekend in the punishing event, coming a day after fellow Kenyan Eliud Kipchoge became the first man to break two hours at the distance when he clocked 1hr 59min 40.2sec on a specially prepared course in a Vienna park.
Kosgei signalled her intentions with an astonishing first five kilometers in 15:28 — so far inside Radcliffe’s world record pace that it seemed she might have ruined her chances out of the gate.
But she settled into a more sustainable rhythm, and powered relentlessly to the finish line.
Her halfway split of 1:06:59 had Kosgei comfortably inside world record pace, and her lead expanded over the second half as her pursuers felt the effects.
The pace-setters dropped away in the closing kilometers, leaving Kosgei to break the tape alone, her arms raised in celebration.
“I’m happy and I feel good,” Kosgei said. “I ran here last year so I knew it was a good course. There was a little bit of wind but it was OK.
“People were cheering all along the course, which gave me more energy.”
Cherono sprints to win
Kenya’s Lawrence Cherono won a men’s race that came down to the wire in 2:05:45 — barely edging Ethiopia’s Dejene Debela who was second in 2:05:46 with another Ethiopian, Asefa Mengstu, third in 2:05:48.
Last year’s winner Mo Farah of Britain was never a factor — finishing a distant eighth in 2:09:58.
Farah was among an early lead group that began to disintegrate around the 10km mark, leaving half a dozen runners, including Cherono, setting the pace.
Kenya’s Bedan Karoki challenged late but faded before the last turn toward the finish to leave Cherono, Debela and Mengstu to sprint for the line.
Karoki settled for fourth in 2:05:53.
It was a disappointing end for Farah to a week that began amid a hail of questions about the scandal surrounding his former coach Alberto Salazar.
US distance running guru Salazar has been banned for four years by the United States Anti-Doping Agency for a string of doping violations.
Salazar has denied wrongdoing, but Nike shut down its Oregon Project running group headed by Salazar, and four-time Olympic gold medalist Farah arrived in Chicago for his defence to find himself again denying any irregularities during his time with the coach.