The 19-year-old US Open champion was thumped 6-0 6-1 in 55 minutes in her first match since recovering from a bout of coronavirus.
Raducanu's defeat came on the same day she secured a ranking of 17 for next week's Australian Open, and she will now be painfully aware of the size of her task ahead.
The British player was over-powered by her opponent, fresh off a run to the final of the Adelaide International last week, and lost the first nine games of the match.
Raducanu flashed a smile after avoiding the indignity of a shut-out in the 10th game but she continued to be let down by a second-serve percentage that barely scraped over 10 per cent.
Raducanu saved the first two match points against her with arguably her two best shots of the match, before succumbing to the inevitable as Rybakina eased into the last 16.
Speaking after the match, Raducanu said she would try not to get too downhearted by the manner of the defeat.
“I think I’ll brush this off,” Raducanu told the official WTA channels. “Yesterday was pretty much my second time playing competitive points for two months. I’m confident that I can brush it off and keep going and keep working.
“I just want to keep putting myself out there. Even if I keep getting knocked down, it’s just about getting back up and basically falling in front. You’re one step better and you learn more.
"I’m just at the start of my first season. My goal is to not get too down or too high."
Raducanu went straight to the practice courts after he defeat in an attempt to work on the parts of her game that are letting her down.
As a seed for the first Grand Slam of the year next week, Raducanu should be given an easier assignment than she got in Sydney, but her current form is starting to be of more concern, losing four of her past six matches since her incredible US Open triumph last September.