The British number one upset Serena Williams in the first round of the Cincinnati Open.
Williams will retire after the US Open, which starts on August 29th, and she lost in the first round of the tournament.
She lacked sharpness and was punished by her opponent.
The winner of the second round will play the victor of the first round.
"I'm so grateful for the experience to have been able to play Serena and for our careers to have crossed over," said world number 13 raducanu, who played superbly with her defence of the Grand Slam title in New York looming.
She has achieved so much and sharing the court with her was an honor.
The British men's number two, Dan Evans, lost 6-3 to the Serbian man.
Evans was unable to maintain his form after reaching the Montreal semi-finals last week.
The second round of the Australian Open begins on Wednesday with a match between Andy Murray and Cameron Norrie on the centre court.
Since Williams announced her retirement last week, there has been a swell of praise for her as a legend.
The respect of the locker room for Williams has been demonstrated in the following days, with world number one Iga Swiatek and another player lavishing praise on the American's achievements and her legacy.
Four-time major champion Naomi Osaka cheered on her hero from the stands as Williams took on a player who was not born when she won her first four major titles.
Williams showed that time can show athletes up on the stages where they used to dominate.
Williams' preparation was affected by an injury in the build-up to the match, which was originally advertised to be played on Monday but then moved back to Tuesday.
Williams was not able to cope with the pressure caused by her opponent's serve and groundstrokes.
After waving goodbye to Cincinnati with a smile, the six-time US Open champion needs to regroup for what she will want to be a more fitting farewell.
She said that being drawn against the American on her farewell tour was a great gift and that her first memory of seeing Williams play was when she was seven years old.
There was no room for nostalgia or charity in her best performance since winning the US Open.
Williams has intimidated her opponents by simply being on the other side of the net, with the power of her serve and powerful game behind her.
She didn't want to be overawed by the event.
Williams' serve was broken to love in the first game, setting the tone for a one-sided match which slowly deflated the former world number one and, in turn, a home crowd desperate to see a national iconfinish her illustrious career with a flourish.
The younger player continued to dominate despite Williams clawing one of the breaks back to give the crowd hope.
In what could be her one and only meeting with Williams, she returned to a similar level to the one she showed in New York last year, despite a frustrating debut season on the tour.
After closing out the opening set with a hold to love, Raducanu raced through the second set by dropping just 10 points and winning with a precise ace down the middle.
"I was afraid from the beginning to the end," said the man.
Serena can return from any situation. I had to keep my focus. I'm happy I kept my cool.
Every single win is important and builds confidence regardless of the level you're playing. You're playing like the best.
I don't want to get too high or low right now.