On day eight of the 2019 championships, two years older at 38, she dredged up a sliver of her old magic to hold off the robust challenge of the world No1 Simona Halep and edge closer to joining Margaret Court on the all-time record of 24 majors.
The 37-year-old American sat out last year's Australian Open after giving birth to a daughter months earlier and dealing with health complications.
She made two more Grand Slam finals past year only to lose the finals at Wimbledon and the U.S. Open but this triumph over the world's leading player felt like another hugely significant step on the road to regaining her old dominance.
Williams' showed her tenacity and will power as she managed to keep the Romanian at bay despite her ruthless attacking onslaught. She then broke for a 4-3 lead and finally was on her way.
In that moment, it was tough not to think back to Williams' last match of 2018 in the US Open final, when the pressure of not performing up to expectations and the emotion of what she perceived as an unfair slight by the chair umpire sent her into a tailspin.
"She's playing very solid this tournament".
"But it's a part of this journey on my way back. It was hard to get through him", the Serbian world number one said.
The turning point came in the sixth game of the third set when Halep clawed three break points but Williams held and took a decisive break in the fifth.
Williams showed that she has not lost the ability to lift herself at the crucial times, and did so again when staring at going a break down in the middle of the decider.
"I will start with a joke".
"It is a miracle that I'm here and that I get to do something that I enjoy".
"I felt like I had been hit by the train in the first set, everything was too fast", Halep said. "I can definitely go to a new level".
"We have had good matches".
Speaking on court, Williams said: "It was a really intense match, some incredible points".
Both players reached the quarterfinals here past year, but Keys had more success at the majors in 2018, when she progressed to the semifinals at the French and US Opens.