The 20-year-old has enthused the likes of Kelly Holmes with her performances this winter but, when it mattered most, she could not deliver as she had wished.
Forlorn on the track in Sopot after she had finished third in her heat of the 800 metres, the Glasgow vet student soon succumbed to tears. With Poland's home favourite Angelika Cichocka advancing automatically into tomorrow's final with the quickest time of 2014, even a promotion into second place following the disqualification of Icelander Anita Hinriksdottir could not save Muir from elimination.
Clipped on the opening lap, Milnathort's finest strode onward but her tactical acumen, so sure of late, let her down. "I lost a lot of ground, it was a case of trying to get back into position and I lost a lot of energy," she confirmed. "Not only does the rest of the field get away from you, but you've got to work twice as hard to catch them up.
"You know you've got to win the race in the first place and you can't afford to lose that much ground. I just didn't have it in that final 200m when I needed it."
Still, there should be plenty of positives to take forward into a summer in which she will surely be a contender in both the Commonwealth Games and European Championships. The inquest conducted with her coach Andy Young should not take long. "It's better," she added, "to make a mistake now than in the future when there is a greater possibility of winning a medal."
Chris O'Hare showed greater resignation as he exited by coming fourth in his 1500m heat. The 23-year-old's form has not quite matched his obduracy of 2013 and although he surged into the lead with 250 metres left, he went backwards as others swept by."You've got to come out and leave it all on the track and come away hoping you have done enough," he said. "I gave it my all, I couldn't have given an extra step. That's championship racing for you."
Nadine Broersen of the Netherlands snared the first gold medal of the championships while the USA's Ryan Whiting defended his shot putt crown. Elsewhere, Tiffany Porter moved into the 60m hurdles semi-finals with victory in her heat, Andy Vernon qualified for the 3000 final by slicing four seconds off his prior best, while 2012 bronze medallist Andrew Osagie progressed into tomorrow's 800m final as a fastest loser.
Dwain Chambers, the champion of 2010, cruised into this afternoon's 60m semis by winning his heat with Richard Kilty heading the standings.
Scottish hopes now rest on the 4x400m relays with Eilidh Child expected to anchor GB&NI's bid to retain the title they claimed in Istanbul two years ago, starting with this morning's semi-finals. While Jamie Bowie is set to feature in a men's quartet that has every chance of securing a place on the podium tomorrow.
Individually, the Inverness Harrier is a step removed from the global elite but he recognises the opportunity which the relays offer, having seen the grand spoils gathered by Lee McConnell over the past decade.
"Lee is a name that you always expected to see in the big races," he said. "She had some big successes individually, including at the Commonwealths. But when you see how long she's been in the GB team, she's been such a big role model for everyone in Scotland."