David Rudisha's comeback after a year out with injury ended in defeat at the International Association of Athletics Federations' (IAAF) Diamond League meeting in Eugene, Oregon.
The 800 metres Olympic champion and world record holder faded to seventh on a night when his fellow Kenyan and Olympic champion Asbel Kiprop also wound up in seventh place in the Bowerman Mile.
Rudisha, who had not raced since winning at last season's IAAF Diamond League in New York on May 25 last year because of a knee problem, was overtaken by six runners in the final 200m of a race won by the 20-year-old Botswana athlete who took silver behind him in the London 2012 Games, Nijel Amos.
After recording a time of 1 min 44.87sec, nearly four seconds slower than his world record of 1:40.91 from the London Olympics, Rudisha commented: "It was tough, but I'm happy to run 1:44."
He added: "Only the last 100m was a little bit tough.
"I started a bit late with my training this year, only in March, due to the knee problems.
"It has been like a crash programme to get ready.
"I need to sharpen up to get competition-ready.
"The body needs competition to build that speed.
"I was tiring over the last 100 metres.
"I'll go back and see how I can build up so I can finish next time.
"But to be out for so long, I am satisfied with my time of 1:44."
Amos, who missed last year's World Championships because of injury, ran 1:43.63 to break the meeting record of 1:43.68 set in 2011 by Abubaker Kaki of Sudan.
World indoor champion Mohammed Aman, the fastest 800m runner in the world in 2013, finished second in 1:43.99 while Kaki finished third in 1:44.09.
In the Bowerman Mile, the Prefontaine Classic meeting's signature event, Kiprop's bid for a record fourth consecutive victory failed as he was passed on the final lap by Djibouti's world indoor champion Ayanleh Souleiman, whose winning time of 3:47.32 was an IAAF Diamond League record and a US all-comers' record, making him the 10th fastest of all time. Kiprop clocked 3:50.26.
"My dream was 3:47 or 3:48," said the 21-year-old Souleiman, who won 800m bronze at last summer's World Championships.
"I was expecting to run close to 3:47 and unfortunately it wasn't my day," said Kiprop, whose compatriot Silas Kiplagat set a personal best of 3:47.88 in second place.
"I don't know. I wasn't comfortable.
"In the last 200m I wasn't responding.
I think this is due to [the fact] that we came from the World Relays last weekend and came straight here."
Kiprop's fellow Kenyan Hellen Obiri, who, like him, was part of a world record-breaking 4x1,500m team at the IAAF World Relays last weekend, fared better as she won the 1,500m in 3:57.05, a US all-comers' record and the fastest recorded in the world this year.
Sweden's world champion Abeba Aregawi had to settle for second in what was her first defeat since August 2012.
Shelly-Ann Fraser-Pryce was a third leading name to suffer the unfamiliar sensation of defeat.
Jamaica's double Olympic and triple world champion, who missed the last Diamond League meeting and the World Relays with a foot injury, finished last in the 200m in 23.06 as the race was won by unheralded home runner Tori Bowie, who clocked a world-leading 22.18.
Blessing Okagbare of Nigeria was second in a personal best of 22.23, while three-time world champion Allyson Felix of the United States took third place in 22.44.
The men's 400m produced a classic race as Olympic champion Kirani James of Grenada defeated former Olympic champion and current world champion LaShawn Merritt of the United States on a photo finish, with both men clocking 43.97, the fastest time run this year.
Another former Olympic champion, Justin Gatlin of the United States, won the 100m in a wind-assisted (+2.7mps) time of 9.76.
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