Muir gave up her spot in the 800m British Olympic group on Thursday in order to concentrate on success in the 1500m, a distance over which she is the current British record holder and sealed European glory back in 2018.
The 28-year-old finished seventh in the 1500m at the last Olympics in Rio five years ago and has reached each of the last three World Championship finals, finishing fourth in London in 2017.
Muir finished third behind Keely Hodgkinson and Jemma Reekie in the 800m at the British Championships last month and subsequently abandoned plans to double up in Tokyo, avoiding a potential schedule of six races in the span of just eight days.
However, Muir then set a new Scottish record in the 800m in Monaco on Friday night, with her time of one minute and 56.73 seconds surpassing Lynsey Sharp’s mark from Rio 2016.
Indeed, Dame Kelly Holmes is now the only British woman to have run the 800m quicker, with her 1995 time of 1:56.21 still the fastest.
While Muir is now second on that list, training partner Reekie is third after running a 1:56.96 on a memorable night in Monaco.
Muir was quoted on scottishathletics.org.uk as saying: “This was my last race before Tokyo, so I just wanted to give it my best and put on paper the sort of shape that I know I am in and I am just so happy with that.
“Having a training partner like Jemma is huge, Jemma has made me a much faster 800m runner. We push each other so much and we’ve both run 1:56, so I have definitely become faster because of her.
“I am going to go home tomorrow, have a week at home before flying out to Tokyo and just train as hard as I can, so I am in even better shape in Tokyo. I want to win a medal, that is all I am focusing on.”
She added: “Running 1:56 is giving me huge confidence for Tokyo.
“I will only be running the 1500m now, I decided that only a couple of days ago, but I couldn’t have asked for a better performance today ahead of the 1500 in Tokyo.”
Muir now holds Scottish outdoor records at 800m, 1,000m, 1500m and one mile.
Elizabeth Bird, another member of Team GB’s Olympic selection, set a British record in the 3,000m steeplechase as she finished seventh in 9:22.80, taking almost four seconds off her personal best.
Additional reporting by the Press Association.