Tennis players over the age of 30 all say the same thing: It’s harder to recover the next day. That’s what 34-year-old Venus Williams seemed to be telling us with every step she took during her first set against Andrea Petkovic on Friday. In her previous match, a win over Victoria Azarenka, Venus had looked sharp, alert, imperious, vintage. Less then 24 hours later, she was hardly able to put a serve near the box, or get her feet out of her own way on her ground strokes, in a 6-1 opening-set loss. Venus finished that set with a first-serve percentage of 38. That's not going to win you many matches at any age.

By the start of the second, as Venus shanked two more forehands and was broken again, I wondered how long she could take the physical ups and downs that have plagued her for the last few years. Just as the thought crossed my mind, of course, Venus showed all of us why she still loves to play, and how far her desire can still take her. Finally, with Petkovic serving at 2-0, 15-0, Venus got off of her heels and onto her front foot. She smacked a forehand return away with disdain, and broke with another big forehand return. She was sluggish no more. By the sixth game, Venus  had managed a fist-pump. By the seventh, she was hitting swing volleys with authority. By the eighth, with Williams up 5-3 and about to level the match at one-set each, Petkovic was bouncing her racquet on the court in anger.

One player had rolled through the first set, the other had righted herself in the second. In the third, they would repeat this pattern two more times. Twice, at 3-1 and 5-3, Petkovic went up a break and Venus looked finished. Twice Venus recovered immediately to tie the score again. But there was no third run this time. Those who have watched her in recent years will recognize the pattern: Venus will fight back against a quality player until she has a chance to win, and then she won’t win. As with her fellow 30-something former No. 1 on the men’s side, Lleyton Hewitt, the fight is there, but the fragility of age is, too. We know which one will win in the end.

In this case, though, credit should also go to Petkovic. Up until 5-5 in the third, the match had resided almost exclusively on Williams’s racquet—she won when she played well, lost when she didn’t. But Petkovic somehow chose this moment, the last moment, the perfect moment, to grab the initiative for the first time. She held at love for 6-5, hit a reflex volley winner to make it 0-30 on Venus's serve, and fired a fearless forehand winner for a 6-1, 3-6, 7-5 win.

This was Petkovic’s first victory over Venus in three tries, and the relief at the end showed. Petko shouldn’t be too relieved. She may have just earned herself a semifinal meeting with Venus’s sister, Serena.