Quarters augur another all-Williams final

Serena and Venus Williams remain on course for a third straight meeting in a Wimbledon final. The sisters advanced in similar fashion Monday, beating hard-hitting opponents in two tight sets.

Serena continues her march Tuesday against China's Li Na in the pick of the quarterfinals, while Venus meets Bulgarian upstart Tsvetana Pironkova. Pironkova and Petra Kvitova make their Grand Slam quarterfinal debuts.

Serena Williams (1) versus Li Na (9)

Li is in good form, having won a Wimbledon tuneup in Birmingham and eliminating Poland's Agnieszka Radwanska (a particularly tough opponent on grass) with ease in the fourth round. Li's power baseline game can trouble anyone.

Serena needs no convincing.

"She always gives me a run for my money," said the world No. 1.

Serena leads 4-1 in their head-to-heads, although all five encounters have been close. The last three sets between the pair, January's Australian Open semifinal included, have gone to tiebreaks. Serena's serve was the difference, and she has produced a generous 39 aces the previous two matches.

Prediction: Williams in three.

Venus Williams (2) versus Tsvetana Pironkova

Venus won't be taking Pironkova lightly, either. At the 2006 Australian Open, the Bulgarian beat Venus 9-7 in the third set.

Pironkova crushes the ball almost as hard as anyone from the baseline, and should be higher than her current ranking of 82nd. She's still struggling with consistency.

"She's talented and she can beat some top players every now and then," Venus said. "So I have to be on the watch for that, continue to play my game as I am now."

Venus stuttered slightly in the fourth round against Aussie Jarmila Groth, who possesses a game reminiscent of Pironkova's. Twice Groth failed to serve out the second set.

Prediction: Williams in three.

Kim Clijsters (8) versus Vera Zvonareva (21)

Clijsters turned it around before it was too late against fellow Belgian Justine Henin in the fourth round, eking ahead 13-12 in their head-to-heads. In winning the second and third sets, Clijsters served well, was consistently good from the baseline and put away comfortable volleys.

The win must have done loads for her confidence, because Henin had won five straight against Clijsters in majors.

Zvonareva reached her first Wimbledon quarterfinal when Jelena Jankovic retired because of a knee injury, trailing by a set and 3-0. What a surprise. The only reason the Russian drifted out of the top 10 was because of an ankle injury sustained last year. Zvonareva is trying to overturn an 0-5 record against Clijsters.

Prediction: Clijsters in two.

Petra Kvitova versus Kaia Kanepi

No one possibly could have predicted this quarterfinal. Kvitova, a lefty from the Czech Republic and ranked No. 62, entered Wimbledon on a three-match losing streak.

Kanepi, a big right-hander from Estonia, boosted her ranking from outside the top 100 to its current spot of 80th thanks to better results the last 1&frac12; months.

Kvitova trounced fourth-ranked Caroline Wozniacki in 45 minutes in the fourth round, and Kanepi, a qualifier, eased past Kvitova's fellow Czech, Klara Zakopalova, in routine fashion, 6-2, 6-4.

Kvitova trails their head-to-heads 2-1.

Prediction: Kanepi in three.