* Italian to face China's Li Na in last eight
* Hard work pays off following extended spell on the sidelines (Adds details, quotes)
Jan 19 (Reuters) - Italy's Flavia Pennetta advanced to her first Australian Open quarter-final with as much relief as delight on Sunday just weeks after suffering a left wrist injury that she thought could rule her out of the Melbourne Park tournament.
Pennetta had reason to be worried after an injury to her right wrist required surgery in 2012, leading her to spend six months on the sidelines, miss last year's Australian Open and watch her ranking drop to 166 by June.
"It was a really difficult moment, because I didn't know what I had," the 31-year-old told reporters after she advanced to the quarter-finals following a 6-1 4-6 7-5 victory over ninth-seeded German Angelique Kerber.
"I had an MRI (scan) five days after which turned out to be pretty good because my ligament and my tendon were fine," she added of the injury she sustained at the Hopman Cup in Perth and forced her out of the Hobart International. "Just a strain."
Pennetta, who has won nine career titles and broke into the top-10 in 2009, found her original injury and subsequent drop in rankings hard to take.
"The feeling I had was completely not good," she said. "I continued to work in a positive way with my team and with everyone, but in the beginning the results were poor.
"I think I had five or six first rounds in a row."
A run to the fourth round at last year's Wimbledon, however, appeared to indicate an upturn in fortunes and improved her ranking and she then reached the semi-finals at the U.S. Open - her best performance at a grand slam.
"Of course the U.S. Open was really important for me," she said. "Everything just changed, because I think I had tried to play all the time and worked really hard."
The hard work has ultimately paid off for Pennetta, who was one of three women aged over 30 to make the fourth round, though at times against Kerber, she was playing like a teenager with nothing to lose rather than a wily veteran of the WTA Tour.
The 31-year-old Italian could do nothing wrong in the first set, moving Kerber around the court and belting winners from both sides to take it 6-1 in 24 minutes.
"The first set was perfect. I was really aggressive. Everything was working pretty good.
"My serve, my return, everything was perfect."
Kerber, who lost in the Sydney International final to qualifier Tsvetana Pironkova, took a short break between sets and somehow managed to stay in the match and break the 29th-seed three times to send it to a decider.
Both players had problems with their serve in the third set with Kerber being broken three times while the Italian was also broken twice - once when she was serving for the match.
"In the third, it was a little bit up and down for both of us. At 5-3, I was serving for the match and I played really bad, I have to say.
"Every point was really long and I was trying to keep it near the baseline while staying on the court.
"But I kept going and tried to stay aggressive, and in the end it worked out well."
Pennetta will next meet China's Li Na, who is just one day younger than herself, in the quarter-finals.
"We are both old," she said while smiling.
"She is so good. She's one of the best players (in the world). Her backhand is unbelievable and her forehand is really good. Physically, she's strong.
"It's going to be really a good fight on the court." (Writing by Greg Stutchbury in Wellington; Editing by John O'Brien)