Serena earns high marks in quarterfinal win

When all else fails, there's Serena Williams. Old reliable came through again Tuesday, slugging her way past China's Li Na 7-5, 6-3 hours after big sister Venus suffered the worst loss of her Wimbledon career, thanks to Bulgarian Tsvetana Pironkova. Kim Clijsters crumpled, not for the first time in a major this year, as did Estonian qualifier Kaia Kanepi.

ESPN.com grades the women's quarterfinalists, with Serena unsurprisingly heading the class.

Serena Williams

The world No. 1 says she has never served this well for a sustained period. Pity her opponents.

Williams threw in 11 more aces against Li, coupled with a strong first-serve percentage (66). She hit 21 winners and committed a mere six unforced errors, one in the second set. Those numbers have something to do with Li -- Serena wasn't underestimating the ninth seed.

The huge favorite now, Serena is saying all the right things.

When told the title was hers to lose, she replied, "No, it's not mine to lose, it's mine to win if I can get it."

Grade (A): The serve was on, and she barely missed

Petra Kvitova

Talk about extremes. The Czech lefty routed third-seeded Caroline Wozniacki in 45 minutes in the fourth round, then went one minute shy of three hours in downing Kanepi 4-6, 7-6 (8), 8-6.

Kvitova, ranked 62nd, simply hung in there. She was constantly under pressure on serve in the second set, saving five break points. In the tiebreaker, Kvitova fended off five match points. Kvitova trailed 4-0 in the third.

"I'm not surprised I'm playing her," Serena said, looking ahead to the semifinals. "Early in the tournament, I saw her playing a couple of rounds and was like, 'Wow, she's doing really well.' She was just hitting the ball so clean."

Grade (A-): Kvitova never gave in

Tsvetana Pironkova

For most, facing a Williams on grass is daunting. But Pironkova said she wasn't intimidated going head-to-head in her maiden Grand Slam quarterfinal with the five-time Wimbledon champion.

One of the hardest hitters in the women's game, 82nd-ranked Pironkova held her nerve early and took advantage of Venus' serving woes to prevail 6-2, 6-3.

"I really pushed her a lot," said Pironkova, who made it two straight wins against Venus. "In the beginning, she started very strong, but then I guess I resisted."

Grade (A-): Even though Venus misfired, Pironkova kept it together

Vera Zvonareva

Zvonareva is currently without a coach, and maybe that's helping the former world No. 5. She's figuring out things by herself on court.

Instead of falling apart after losing the opening set to Clijsters, the Russian rallied 3-6, 6-4, 6-2 to reach a second Grand Slam semifinal.

"I was trying to stay [focused], keep my concentration," Zvonareva said. "Nothing else was bothering me. I wasn't noticing anything around."

Zvonareva produced a clean 16 winners to go with nine unforced errors in the final two sets.

Grade (A-): Zvonareva hung tough mentally

Li Na

One game realistically cost Li in her scrap against Williams. At 5-5, she blew a 40-0 lead on serve, sunk by back-to-back double faults.

Before then, it looked as if Williams and Li were about to contest a fourth straight tiebreak in their head-to-heads.

Predictably, Li dipped in the second set.

Grade (B): Not much separated the two

Kim Clijsters

What happened to Clijsters from the second set onward?

Yes, Zvonareva picked up her game, and yes, Clijsters just returned from a foot injury. But the Belgian disintegrated. Again. Recall that the reigning U.S. Open champ got pummeled at the Australian Open by Nadia Petrova.

It seems as though when the pressure rises, Clijsters wilts.

"This is my first time back at Wimbledon" since coming out of retirement," Clijsters said. "I haven't been back for a year yet. Obviously, it takes time."

No doubt Clijsters is looking forward to the U.S. Open series.

Grade (D): Another Grand Slam meltdown

Kaia Kanepi

When the dust settles, Kanepi said, she'll look back on this tournament with fondness. Getting to the last eight and resurrecting her career are indeed pluses.

However, Kanepi blew one of the match points on a double fault and led 5-3, 30-0 on serve in the third set. Kanepi later denied it, but it appeared as if she tired. She went for high-risk shots, and her potent serve deserted her.

Grade (D): How many more chances did she need?

Venus Williams

As much as one needs to credit Pironkova, Venus lost the match. (The writer said it, not the player.)

Williams hit a staggering 29 unforced errors (a lofty number for two sets on grass), served at less than 60 percent, and won 21 of 40 points at the net, where she's usually so dominant on the sport's fastest surface.

"I definitely made too many contributions to her," Venus said.

Grade (F): A shocking display