Britain’s Emma Raducanu loses on her return to court for the first time since her US Open triumph.
World number 22 Raducanu was seeking an 11th successive victory, having won the US Open last month as a qualifier.
But Sasnovich, ranked 100th in the world, beat the 18-year-old 6-2 6-4 in round two of the BNP Paribas Open.
Raducanu, who received a first-round bye, is next scheduled to play at the Kremlin Cup in Moscow in 10 days’ time.
The Briton was competing without a permanent coach having ended her partnership with Andrew Richardson, whom she has known since the age of 10, after her US Open success.
Speaking after the match, she told BBC Sport: “We had some great memories together – at the US Open and also previous to that – but I’m looking forward to the next chapter and I think that what I am doing right now is definitely the best for me and my tennis.”
Raducanu made an excellent start against Sasnovich – holding to love in the opening game in front of a supportive crowd of about 4,000 for the night session.
But a loose service game followed, and with her 27-year-old opponent playing some excellent defensive tennis, the US Open champion was forced into too many errors.
Raducanu seemed anxious to close out rallies quickly on the slow court, and Sasnovich broke again to love and took the opening set.
It was the first set Raducanu had lost since a three-set defeat by Clara Tauson in the final of the Challenger event in Chicago on 22 August.
Sasnovich broke first in the second set too, as she showed off the attacking side of her game.
But two double faults followed in the next game and with Raducanu beginning to find some composure, the teenager was able to open a 4-2 lead.
The improvement was short-lived, however, and Sasnovich won the final four games of the match to complete a straight-set victory.
The Belarusian will play two-time Grand Slam champion Simona Halep in the third round on Sunday.
Raducanu said: “I think Aliaksandra played a great match. She was better than me today so she deserves to win.
“I’m kind of glad that what happened today happened so I can learn and take it as a lesson.
“There’s going to be disappointment after any loss. I didn’t go in there putting any pressure on myself because in my mind I’m so inexperienced that I’m just taking it all in.”
