Six months out doesn't usually help a player - Pennetta was out that long between August 2012 and February 2013 with the wrist injury - but last year she came back with a vengeance at the US Open, reaching her first Grand Slam semifinal, and now she's conjuring up the same magic in Melbourne.
The No.28-seeded Italian fired 43 winners - and regrouped after letting a 5-3 third set lead dissipate to 5-all - to stun the No.9-seeded Kerber in an hour and 52 minutes on Rod Laver Arena, 61 46 75.
"I think the first set was perfect," Pennetta told reporters in her post-match press conference. "I was really aggressive. Everything was working pretty good. My serve, my return, everything was perfect.
"In the second set I was waiting a little bit more and she started to play much better - of course, it's normal. I had a few chances in the second set and didn't make it. And then in the third it was a little bit up and down for both of us - I served for the match at 5-3 and played really badly, I have to say.
"But I just tried to keep going and tried to be aggressive, and in the end it was working pretty well."
Pennetta is now through to the fifth Grand Slam quarterfinal of her career. This is her first outside her beloved US Open - she fell in the quarters there in 2008 (to Dinara Safina), 2009 (to Serena Williams) and 2011 (to Kerber) and then went one further to the semis in 2013 (falling to Victoria Azarenka).
Did Pennetta believe she would challenge for Grand Slams again after the surgery? "No," she said. "The feeling I had was completely not good. I kept working well with my team and everyone, but in the beginning the result was not good at all. I did like five or six first rounds in a row. But everything has just changed, because I think I was there and trying to play all the time and just working really hard.
"So this is amazing. It's always nice to be in the quarterfinals in a Grand Slam, and it's the first time for me here, and I really enjoy playing here. Today was a really good match and I'm just very happy."
"We didn't play in the last two or three years, so it's going to be a completely different match," Pennetta said. "She's so good. She's one of the best players in the world. Backhand, unbelievable; forehand, really good; physically she's so strong. I think it's going to be a really good fight on the court."
Pennetta and Li are in the rising group of 30-somethings on the tour - why is it such a strong group?
"Well, when you're older, you start to have different goals and try to enjoy life a little bit more. When I was young it was really easy to stay away from my house a few months and not see my parents, but now everything is starting to be difficult. I love to come back. I love to spend time with them. And so I'm here and trying to play my best tennis, and trying to enjoy every moment. I'm really trying my best."