Cheptegei took more than 22 seconds off his personal best as he averaged 61-second laps over the 5,000m.
Uganda's Joshua Cheptegei spent much of his coronavirus-induced lockdown painting a primary school and helping out in his grandparents' vegetable patch in his homeland.
But a relatively quick easing down on restrictions by the Ugandan government in June meant Cheptegei could get serious with his training under coach Addy Ruiter, and on Friday, he showed how successful that had been.
Metronomic pacing and perfect conditions at the Stade Louis II saw the 23-year-old light up the Monaco Diamond League, the first regular top-flight international one-day meet since the advent of the COVID-19 pandemic.
Cheptegei wiped almost two seconds off Ethiopian legend Kenenisa Bekele's 16-year-old world record time over the 5,000m as he clocked an astonishing 12min 35.35sec.
"I've learned that anything is possible, if you have the right mindset and believe," the Ugandan said.
"I really thank Kenenisa so much for inspiring me when I started running. He has always been a big inspiration and motivation to me.
"This record is a special moment for me and I like to thank Kenenisa for his inspiration."
Remarkably, Friday's outing was Cheptegei's first competitive appearance since February, when he smashed the world 5km road record, also in Monaco.
Those two world records in a COVID-hit season followed an outstanding 2019 when he secured not only the world cross country title in Denmark, but also the world 10,000m title at the Doha world championships.
Cheptegei took more than 22 seconds off his personal best as he averaged 61-second laps over the 5,000m, staged in front of a masked crowd limited to 5,000 because of virus-related health and safety restrictions.
Paced through the early stages by Roy Hoornweg (2:31.87 at 1000m) and Matthew Ramsden (5:03.77 at 2000m), Cheptegei hit the front at halfway and continued the metronomic pace, passing through 3000m in 7:35.14 and then upping the pace slightly with a 2:30 fourth kilometre.
He was then out on his own, but managed a final effort of 59.64sec over the final lap to guarantee himself a record that could stand for some time to come.
"I think Monaco is a special place and it's one of these places where I could break the world record," said Cheptegei, who spurned his usual European base for his home in Kapchorwa, Uganda, for the pandemic lockdown.
"It was both good and bad," he said. "It allowed me to spend more time with my family, but at the same time we are missing the sport so much.
"I did some gardening at my grandparents' house. But mainly, I worked at my school in town. It's a primary school, and I worked on some renovations there, like painting the walls."
Uganda, Cheptegei added, didn't have many positive COVID-19 cases.
"About two months ago, the government relaxed the lockdown," he said. "At first, we had individual training but we could quickly train in small groups.
"I honestly really missed competing. It's something I love doing, it's in my blood."