Serena Williams' stubbornness helped her win 21 Grand Slam events and remain the No. 1 player in the world at age 34. But it also may keep her from adding to her legacy.

A day after withdrawing from her first match at the Hopman Cup, Williams retired during the second set of her match against Australian Jarmila Wolfe on Tuesday, citing a sore left knee, according to Yahoo Sports.

Three-Month Layoff Doesn't Help Serena Williams' Knees

"I just have some inflammation that's been going away very slowly," Williams said after the match. "It's still there, it's going away, but just needs a little more time. A little rest, a little treatment."

Serena added that she did not believe the injury would jeopardize her appearance at the Australian Open, but she included a minor qualifier in her comment.

Serena Williams Is Attempting To Foster Sexy Image

"Speed is my strength so I definitely think I will be able to get it right for Australia," she said. "I only think positive and I know I'll be okay."

Williams revealed that she has conducted grueling offseason workouts in preparation for the upcoming season.

"I've been training really hard during the offseason and really pushing myself beyond the limits," Williams said. "I just think a day off or two will make a world of difference."

Williams, however, had better proceed cautiously. She and her coach, Patrick Mouratoglou, conceded that the knee had bothered her during her U.S. Open run that ended with a semifinal upset at the hands of Roberta Vinci.

Her loss to Vinci may have prodded her to work harder at the expense of her health. She may be trying to redeem herself in Melbourne, though she might've been better off skipping the event to help her knees heal.

Mouratoglou compared Williams' knee issues to those of Rafael Nadal. But at 34, the issues could be much more problematic for Williams. If training hard causes her knee pain, she may not be able to get through two-week Grand Slam events. And if she eases up on her training, she may not be prepared physically for a two-week grind.

The troublesome knee could keep Williams from successfully defending her Australian Open title, but it could also mark the beginning of a body breakdown that keeps Serena stuck at 21 Grand Slam titles -- one behind Steffi Graf and three behind Margaret Court for the all-time record.

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