Kenya's Kelvin Kiptum set a world record to win Sunday's Chicago Marathon men's title in an unofficial time of two hours and 35 seconds.
The 23-year-old clocked 2:00:35 to eclipse Eliud Kipchoge's previous mark by 34 seconds.
Within the first couple of kilometres, seven runners and a number of pacemakers had formed a lead group.
And just over 5km into proceedings, only London Marathon winner Kiptum and Daniel Kibet Mateiko remained in touch with a sole pacemaker.
The pair were well outside world record pace when they reached halfway in 1:00:48, but it was not until Kiptum kicked away with 10km to go that Kipchoge's mark from Berlin last year came back into view.
A blistering 5km of 13:51 from 30-35km suddenly put Kiptum on track to go under 2:01 and he continued to stride on at the front.
It was Kiptum's third victory in as many starts over the 26.2-mile distance. He won on his debut last December at Valencia and then in London Marathon last April.
It marked the third time a men's world record had been set on the streets of Chicago but the first time since Morocco's Khalid Khannouchi in 1999.
Before the finish, Kiptum was waving and blowing kisses at spectators before raising his arms in triumph at the finish line.
The old Chicago Marathon record of 2:03:45 was set by Kenyan Dennis Kimetto in 2013.
Sifan Hassan was on course to emulate Kiptum and repeat her London success in the women's race.
She and two-time reigning champion Ruth Chepngetich completed the first 10km in 31:05, 40 seconds quicker than Tigst Assefa when she set her world record in Berlin a fortnight ago.
With three male pacemakers for company, the pair continued to set scorching splits and go further clear of the rest of the field.
By 20km, Hassan - who won two medals at the World Athletics Championships in Budapest six weeks ago - had dropped back from Chepngetich who reached halfway in 1:05:42, at least 45 seconds inside world record pace.
But just as she did in her winning debut in London, Hassan recovered from having looked in trouble and - just after the 25km mark - started to move away from her Kenyan rival.
While she soon lost touch with Assefa's world record time, the Dutchwoman was well clear with 5km to go as she closed in on her second victory in two marathons. [Additonal reporting by Olympics]