This year's US Open hasn't been short of shocks, surprises and drama.
Both have breathed new life into this year's tournament, giving a casual throwback to years gone by when both used to sweep their opponents off the court.
It is what took both to the pinnacle of the sport, becoming world number one on the men's and women's circuits respectively.
The Italian hasn't looked his best though over the American hard-court summer, with opening-round defeats during the Canadian Open to eventual winner Pablo Carreño Busta, and to Frances Tiafoe in Cinccinati.
The past takes on the future as the two Americans lock horns.
Keys is one of a small cluster of players to have beaten world number one Iga Świątek this year.
The 27-year-old will be the underdog in this match-up, though, considering the form off Gauff.
The youngster is often vocal about her admiration for idol Serena, who herself won her first Slam in New York.
So expect to see a pumped up Gauff in this clash.
Serena Williams vs Ajla Tomljanović
40-year-old Serena seems to be riding high on the pre-retirement party atmosphere and soaking it all up.
The manner of that victory showed that the six-time US Open champion can still beat the best players in the world on her day.
However, Croatian born Australian Ajla Tomljanović is the younger of the two, and will be fresher given Serena's doubles commitments with sister Venus.
But with the Big Apple all behind their icon and attempting to push her onto more battles this fortnight, it is anyone's game.
Will Serena's bid for a record-equalling 24th Grand Slam title remain alive on Friday night? The tennis world holds it's breathe.