The seven-times winner has been away from tennis for more than a year after the birth of her first daughter.
But she didn't let the effects of a lack of match fitness show as she came from a set behind to beat Italy's Camila Giorgi 3-6 6-3 6-4.
And Williams said: "Every time I play Giorgi she plays that level. She always plays like that and I knew it wouldn't be easy.
"She always gives 100 every match and I don't know what I did. . . I just kept fighting.
"I feel good and like I did better, this is only my fourth tournament back so I don't feel pressure.
It was Giorgi's first ever Grand Slam quarter-final, but she shocked the 23 times Slam winner by taking the first set.
At that stage it looked as if there would be yet another upset in a year which has seen the top ten seeds crash out of the tournament already.
But Williams drew on her experience to ensure she didn't panic at the first set loss, as she gradually wore down the spirited play of Giorgi.
"I feel like I am back, but still have a long way to go to be where I was."
The 36-year-old will now face experienced German Julia Goerges in the last four as she attempts to crown her comeback with a brilliant eighth title.
Goerges, seeded 13th this year, was also forced to come from behind to see off the challenge of Kiki Bertens.
Belgian star Bertens had won the opener comfortably, but Goerges took a tight second and then stormed through the decider to record a 3-6 7-5 6-1 win.
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