A good-and-bad day for the Williams sisters at Wimbledon

Reporting from Wimbledon, England — Serena Williams straightened out the very crooked women’s Wimbledon quarterfinals Tuesday. The defending champion’s uncomplicated 7-5, 6-3 win over ninth-seeded Li Na was, in fact, the only occurrence that made sense.

Tsvetana Pironkova, a Bulgarian ranked 82nd in the world, considered herself surprised to have eliminated second-seeded Venus Williams, 6-2, 6-3.

It was a thorough and unexpected beating for a five-time Wimbledon champion, and more surprising even than the exit of eighth-seeded Kim Clijsters, the 2009 U.S. Open champion, who was put out by 21st-seeded Vera Zvonareva, 3-6, 6-4, 6-2.

In the fourth quarterfinal, Kaia Kanepi missed becoming the first Estonian to make the semifinals of a Grand Slam event after failing to convert five match points and losing, 4-6, 7-6 (8), 8-6, to unseeded Petra Kvitova of the Czech Republic.

Serena Williams, seeded No. 1, will play Kvitova in Thursday’s semifinals and Zvonareva will meet Pironkova.

As former player and ESPN analyst Pam Shriver said, “The only way for Serena to lose this tournament is if her serve betrays her, but she looks so locked in right now with that. It’s the best shot in women’s tennis.”

While Serena moved ahead, older sister Venus fumbled with her forehand and too often failed to catch up to the nerveless groundstrokes of the so-far undistinguished Pironkova.

Venus seemed muddle-minded during the 85-minute match, with no sense of how to get out of her sudden funk. The 30-year-old author (she released a book, “Come To Win,” this week) and clothing designer (she wears her own label, EleVen) seemed far removed from her five Wimbledon titles.

“I think I missed all shots today,” she said. “Forehand, volley, backhand. You know, if there was a shot to miss, I think I missed it.”

But she bristled at the suggestion that her motivation or desire for tennis might be diminishing.

“Why wouldn’t I want to pursue this?” Venus said. “I’m pretty good at it most days. Today I didn’t seem to be the best tennis player but for the most part I rock and roll this game.”

Serena rocked and rolled with her 11 aces and only six unforced errors, an untouchable combination of power and precision against the 28-year-old Li. And although Serena is often hard on herself, she admitted her serve has been a reliable companion so far in this Wimbledon.

“I usually serve well here but this is the first time I’ve ever served this well so consistently here,” Serena said.

Venus had nothing consistently go well against Pironkova. She committed 29 unforced errors and hit her forehand with little conviction, as if she were still discovering how that stroke works.

“I didn’t do myself any favors,” she said. “I felt like she played solid but if I hadn’t contributed to her effort, I’m not sure it would have gone as well.”

Clijsters was disappointed in her inability to pressure Zvonareva. The Russian is often a nervous finisher and on Tuesday buried her head in a towel at each changeover as if trying to hide from her possible triumph.

“Vera served well at important points,” Clijsters said, “but I never really made her work for it all that much. That’s the most disappointing thing for me.”

At the end, Clijsters and Venus Williams sounded very much alike.

diane.pucin@latimes.com

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