The man asked the woman for a cigarette light as she walked on a path at Wychall Reservoir, Birmingham.
Moments later he "grabbed her from behind, dragged her to the floor and assaulted her", police said.
The man who carried out the attack on 18 November was smartly dressed in blue trousers and black pointed shoes and spoke with a Scottish accent.
He was also described as white and in his 30s or 40s. The woman also recalled that her attacker smelled strongly of aftershave.
The assault took place on a path running between Popes Lane and Wychall Road at about 17:00 GMT.
"This is a shocking attack on a woman by a stranger," said Det Con Fay Mason. "We need to identify her attacker as soon as possible."
Anyone with information was urged to contact West Midlands Police.
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