The 23-time Grand Slam singles champion had already won four of those titles before Raducanu was even born in November 2002 and the pair had never met previously, with Williams having spent most of the 14 months since Raducanu’s Wimbledon breakthrough last summer on the sidelines.
”I can't believe I just played Serena Williams," Raducanu said. ”It's something that I think I'm really fortunate to have been able to do, and for our careers to have crossed when there's such a big [age] gap and watching her growing up, it was an amazing experience to just play her.
“You have to cherish the moment and you’re going to have this memory for the rest of your career.
"Everything that she's achieved is so inspirational and it was a true honour to share the court with her.”
Whatever the outcome, it was always going be difficult not to read too much into this headline encounter.
Option one was that Williams would sink the US Open champion to ignite hope of a dream send-off at that tournament later this month, where the 41-year-old will have one final shot at equalling Margaret Court’s record of 24 Slam titles.
The alternative, that Raducanu would down arguably the greatest tennis player of all-time and signal a timely return to form ahead of her return to Flushing Meadows and the defence of her crown.
I’m obviously really proud of myself, and it does give me confidence.
The 19-year-old has not won a tournament since her fairytale of New York, but is looking to build momentum with under a fortnight to go until the start of the final Major of the year.
There were caveats: this was just the fourth match of Williams’ season and it had already been pushed back 24 hours, officially for scheduling reasons, but with rumours swirling of a knee injury concern for the American. But still, this was a major boost for Raducanu, who made just one unforced error and at times played close to the level that saw her charge through qualifying to that unexpected US Open success 12 months ago.
”Playing Serena and managing to win, I'm obviously really proud of myself, and it does give me confidence,” she added.
”Each of these wins kind of build confidence regardless of the level you're playing, every single win counts. But of course this one more so, because you're playing, like, the greatest. But I'm not trying to get too high or too low right now.”
Raducanu broke the Williams serve to love in the opening game and, while back-to-back aces saw the home favourite battle back into the first set at 4-3, the Briton held her nerve before running away from a resigned opponent in the second.
Last week at the Canadian Open, Williams had secured her first victory of the season, against Nuria Parrizas-Diaz, before falling to Olympic champion Belinda Bencic having announced her retirement plans in Vogue.
The second stop on what is a short farewell tour has already come to an end and it remains to be seen whether Williams will play at either of the two remaining tune-up events, in Cleveland and Quebec, or else head straight to the US Open.
Organisers in Cincinnati had planned a post-match tribute but Williams made a swift exit after planting a forehand into the net to confirm Raducanu’s victory, declining an on-court interview.
Next up for Raducanu, meanwhile, will be a second-round clash against two-time Major winner Victoria Azarenka on Wednesday night, but another former US Open champion, Naomi Osaka, is out after suffering a straight-sets defeat to China's Zhang Shuai.
There was more grim news for the home crowd as teenager Coco Gauff was eventually forced to retire hurt while trailing 7-5 1-0 after rolling her ankle in the opening game of her first round match against qualifier Marie Bouzkova.