Ciara Mageean said she "just didn't feel myself" after finishing 11th in the 1500m at the Diamond League Final in Eugene.
With Faith Kipyegon again dominating, Mageean struggled from the off as she clocked 4:03.09 - nearly seven seconds shy of her personal best set last week.
"Not the race I wanted out there today," said Mageean on Instagram.
"But I've had an amazing season this year so now it's time to have a wee break."
Kenyan star Kipyegon was a class apart as she followed the pacemaker to be way clear of the pack by the 800m mark which was reached in 2:05.26.
During the penultimate lap, Kipyegon remained on course to break her own world record of 3:49.11 set in Florence three months ago but she lost pace in the closing stages as she had to be content with setting the fifth fastest time in history with a 3:50.72 clocking.
Ethiopia's World Championships silver medallist from last month Welteji Diribe was once again the closest to Kipyegon as she ran a personal best of 3:53.93 with Britain's Laura Muir third in a season's best 3:55.16.
Muir had edged out Mageean in last week's Diamond League meeting in Brussels when the Irishwoman had improved her national record to 3:55.87.
Ethiopia's Freweyni Hailu finished fourth in a personal best of 3:55.68 with Linden Hall (3:56.92) setting an Australian record in fifth as Ethiopians Birke Haylom (3:56.98) and Hirut Meshesha (3:57.53), Australia's Jessica Hull (3:57.57), Britain's Melissa Courtney-Bryant (3:59.57) and American Cory Ann McGee (4:01.28) also finished ahead of the Northern Irishwoman.
Brian Fay is the other Irish athlete in action over the two days of Diamond League Final action, which doubles as the Prefontaine Classic.
Fay, 24, will compete in Sunday's 3,000m after a season in which he has set a new Irish 5,000m record.
The Dubliner clocked 13:01.40 in July to take more than two seconds off Alistair Cragg's 2011 mark although the World Championships proved a disappointment as he could only finish 16th in his semi-final in 13:42.16.
With Jakob Ingebrigtsen schedued to run on Sunday after winning the mile on Saturday in the third fastest time in history, a fast pace looks likely and Fay may target his personal best of 7:36.89 set in Italy last week which moved him to third on the Irish all-time list behind national record holder Mark Carroll (7:30.36) and Cragg (7:32.49).