From start to finish the No.4-seeded Li was on complete fire, blasting 23 winners to just 17 unforced errors against one of the fastest players in the game. Meanwhile, the No.28-seeded Pennetta couldn't get too much going on the day, hitting 14 winners to 27 unforced. It was all over in 67 minutes, 62 62.
"I prepared at least one and a half days for this match," Li said. "I knew she was going to be a tough opponent - I had played her four times before, and every time was tough. But I don't really know what happened today. I just played really well. I just prepared really well before I came to the court.
"I'm really happy. Everyone wants to win a Grand Slam, but you have to be in the semis first, right?"
Li is now through to the semifinals of the Australian Open for the fourth time in her career, all in the last five years as well - she was a semifinalist in 2010 (falling to Serena Williams), a runner-up in 2011 (falling to Kim Clijsters) and again finished runner-up in 2013 (falling to Victoria Azarenka).
But she was almost out in the third round this year - she was down match point against Lucie Safarova, and she didn't really have a say in that match point, as Safarova barely missed a backhand down the line that was out of reach. She had a talk with her coach, Carlos Rodriguez, after that fateful day.
"He didn't tell me what I should do, he just asked me, 'What do you want? Why do you do this?' After three questions, I felt like he was already punching me down. So I was like, 'Okay, I have to do something, otherwise he'll be angry.' The next day I started preparing. The next day I started doing what I said I would do. I'm always promising I'll do things, but I never do. But this time I did.
"I think this is why things have turned around since then."
Pennetta was playing the quarterfinals of the Australian Open for the first time. She had been to the quarterfinals or better at Grand Slams four times before, but all of those runs came at the US Open.
"I'm happy about this tournament. It was a really good tournament," Pennetta said. "I played a few really good matches and I always enjoy playing here in Australia, so I have to be happy about it.
"Of course, I'm a little bit upset now because I would have liked to play a little bit better today, but next week I have another tournament and everything's going to start again, so I'll just keep going."
And how did the Italian feel about Li's form on the day? "It was perfect for her - since the first ball, I think she played really well today," she said. "She's improved her game a lot. She's more consistent with the forehand, and her serve is working really well. She was making no mistakes at all today.
"She's one of the best players. She's a Grand Slam champion. That's what we're talking about."