That the 31-year-old 400 metres runner is at the global showpiece is an inspiring story of will, courage and a burning desire to compete.
A little over a month before last year's Olympics, Williams-Mills was diagnosed with breast cancer. Her mind in turmoil, she still ran in London and finished fifth.
A few days later she won bronze in the 4x400 relay before returning home to steel herself for her fight for survival.
Surgery, the first of four trips to the operating theatre, came immediately. Williams-Mills first had a small lump removed and later underwent a double mastectomy. Her last surgery was on January 18 and 20 days later she began her journey back to the track.
Initially reticent to talk in public about her disease - her husband Jameel, family members and a handful of close friends only knew what she was going through during the Olympics - she is determined to spread her message of hope.
"For a year my friends and family asked me 'are you ever going to tell your story'," she told Reuters during a break in training on Thursday at a warm-up track close to the Luzhniki stadium.
"I struggled with it because I didn't want people to look at me differently. They encouraged me though and said my story could help so many people.
"When I went public with it - to get the response and support that I have got - people saying I was an inspiration to them...I feel so blessed.. you know, to know I helped somebody."
The five-times world and three-times Olympic medallist has yet to get the all-clear from her doctors - she still needs regular check-ups - but believes that where there is a will, there is a way.
SO FEARFUL
"I want them to know that I don't have a 9-5 job but this takes so much out of my body to even be on the track, to compete at this level. I want them to know it's still possible, that their dreams can still come true," she said.
"For me, at 30, I didn't know I would be fighting breast cancer. There are so many people out there that are still fighting breast cancer... so many people out there are so fearful of what's going on in their life because they don't know if they can live to see tomorrow."
Her upbeat state of mind is a stark contrast to the fug she found herself in before she got on the plane to London.
"In the back of my mind I didn't know if it was going to be my last race. When you learn you have cancer your mind goes somewhere else."
Appointed a team captain, Williams-Mills betrayed little of the emotional strain she was under in the British capital, her engaging smile at press conferences giving nothing away.
"To say hi to everyone, saying I was great when inside I knew it wasn't great, to know what I had to come back to face...
"Physically I was there but mentally I wasn't... You start thinking about what you go through, not running a race."
Lifted by her husband's encouragement and support, the Olympics proved a short distraction. But reality soon hit home.
"When that last 4x400 was done and I was on the plane heading back to Orlando I cried. I cried because I knew what I was going home to face," she recounted.
Her battles left her drained but "up for the challenge" of regaining competitive fitness - and her year's best time of 50.01 in Kingston on June 23 was evidence that the spark was coming back.
Admitting it would be "tough" to produce her best in Moscow, just getting to the Russian capital was a victory.
"When I started my whole surgery I didn't know I was ever going to make the Jamaica team for the world championships.
"I've never gone into a season with this least amount of preparation. But I was up for it."
(Editing by Alison Wildey)
