The British number one looked in fine shape after an excellent first set, but she was unable to maintain that early momentum and slipped to a 3-6 6-1 6-1 defeat.
Sasnovich, the world number 47 from Belarus, had emphatically won their only previous meeting, 6-2 6-4 in Indian Wells last October in Raducanu’s first match after winning the US Open.
But a pumped-up Raducanu broke her opponent in the fifth game of the opening set. Sending Sasnovich wide whenever possible, the 19-year-old from Kent unleashed a couple of fearsome backhands and finished the job with a blistering forehand.
A second break wrapped up the opening set before a wobbly start to the second, including a pair of double faults - her first of the match - saw Sasnovich snatch a double break and a 4-0 lead.
Raducanu’s winners had dried up; she hit just four to Sasnovich’s 19 in a stinker of a set as the 28-year-old from Minsk levelled the match.
A reset was required and, after a quick departure from the court, Raducanu reached 30-30 on the Sasnovich serve only to dump a backhand into the net, prompting to cover her face with her racket in frustration.
Nevertheless, the fist pumps were back as she saved a break point to hold for 1-1.
But game three was to prove pivotal, Sasnovich fending off five break points amid seven deuces to hold, and moments later Raducanu mistimed a forehand and found herself a break down.
Sasnovich was now in complete control with Raducanu unable to stem the flow of winners, and she went on to run away with the set and reach the third round for the first time.