In one of the weakest women's fields at the worlds, established skaters such as twice former champion Mao Asada, four-times European gold medallist Carolina Kostner and Four Continents winner Ashley Wagner had been expected to fight it out for the medals.
Leonova, however, hijacked the competition with her storming performance to the Pirates of the Caribbean soundtrack, nailing all her elements including a soaring triple toeloop-triple toeloop combination to earn a personal best score of 64.61.
She flawlessly floated from one manoeuvre to the next and ended the two minute-50-second exhibition with a menacing glare as she pretended to slit her own throat with her gloved hand.
By the end of the competition, her rivals were the ones who were feeling the pain of disappointment at being ambushed by a relative unknown.
The Russian led another unexpected medal contender, 17-year-old Kanako Murakami of Japan, by 1.94 points while Italy's Kostner stood third with 61.00 after doubling a scheduled triple loop.
The omens also looked good for Leonova, who said: "I have already competed here six times (in lower-tier competitions) and I never left Nice without a medal."
Asada will have to rely on the leaders hitting the self-destruct button during Saturday's free skate if she is to have any chance of topping the podium as she needs to make up a deficit of more than five points.
The Japanese skater has had a rollercoaster few months. She looked to have reclaimed her best form at the start of the season but then had to deal with her mother's sudden death in December.
On Thursday she was the only woman of the 30 competitors to attempt the triple Axel but her gamble backfired. Dressed in a sequined, sea-blue dress, she resembled a floating wave as she fell over and slid across the ice on her hands and knees, incurring a one-point deduction.
"I was more nervous than happy or excited about the competition today," Asada, the first woman to land three triple Axels at an Olympics, told Japanese reporters.
"Since coming to Nice, I had not felt confident about pulling off the jump but attempted it anyway as otherwise I might regret it.
"I'm now going to go away and think about the jump as I'm not sure if I will try it in the free skate."
Wagner will also have a lot to think about after her hopes of ending U.S. women's six-year medal drought all but vanished when she failed to pull off her opening triple-triple combination.
She stepped on the ice shimmering in a red, halter-neck dress but the sparkle went out of her routine within the first 15 seconds of her performance and she trailed in eighth.
"Today wasn't my strongest performance but it was a long time period between my warm-up and my actual skate. So my body had cooled down by the time I tried to do the triple-triple," said Wagner, who beat Asada last month to the Four Continents crown.