The Williams sisters have been unbeatable in doubles so far. But only one of them can win the singles title. Toby Melville/Reuters Jon Wertheim's Mailbag Jon Wertheim will answer questions from SI.com users in his mailbag every Wednesday. Name: Email: Hometown: Question: SEND Follow Jon Wertheim on Twitter

After six days of play, here are our midterm grades from the 2008 version of Wimbledon:

A

Rafa-Roger III: So far, so good.

Venus-Serena III: So far, so good. (And they're cruising in the doubles draw.)

WTA depth: Six players outside the top 100 reached Round 3, including giant killers Jie Zheng and Alla Kudryavtseva, who beat Ana Ivanovic and Maria Sharapova, respectively.

Marat Safin: Fires up one for the memory banks by beating Novak Djokovic; also impressive, he returns two days later and wins a tight four-setter.

Nicole Vaidisova: Apparently shaking off a mean slump, Czech reaches round of 16.

Bethanie Mattek: American veteran best known for her outrageous attire, comes to the one event where she's required to dress conservatively. And she has the best Slam result of her career. Coincidence?

Tamarine Tanasugarn: Wielding the Thai Stick, veteran reaches second week for the seventh time in her career.

A-minus

ESPN: Generally solid first week, made all the better by the return of Brad Gilbert. But a host of readers complain that the "score graphic" on the top of the screen is confusing. Just playing middleman here.

B

Nathalie Dechy: Comes within a point -- a lousy net cord, in fact -- of upsetting top-seeded Ivanovic. But can't slam the metaphorical door.

The greenskeepers: On the one hand, the (allegedly) slower grass makes for a more democratic surface. On the other hand, let's not hope we're heading toward a uniform surface speed at all four majors.

Ernests Gulbis: Loses to Nadal but is there any doubt this guy is headed for the top 10?

C

Novak Djokovic: For a guy who's been heaping dirt on the Federer grave, the Djoker ought to be doubly embarrassed losing in Round 2. Remind us again: who's vulnerable?

Ana Ivanovic and Maria Sharapova: In a titanic upset, the top seed and the 2004 champ fall in week one, with remarkably similar flat efforts against players outside the top 100.

American men: We expect early flameouts at the French. When the only male to reach Round 3 at Wimbledon is Bobby Reynolds, we got issues.

David Nalbandian: You're a former Wimbledon finalist. In the past year, you've beaten Roger Federer, Rafael Nadal and Djokovic. And you lose your first match in straight sets to Frank Dancevic?

Justin Gimelstob: Dozens of you referenced his unfortunate remarks. The full disclosure, of course, is that Justin has been writing for si.com, which makes this all the more awkward. The remarks themselves are thoroughly indefensible. But let me say this in his defense: he's always been a beacon of candor, one of those athletes who speaks honestly, regardless of whom he might offend or what collateral damage might arise. This doesn't, of course, give him license to say anything, but -- and I feel similarly about Charles Barkley -- it tempers at least some of the outrage when he crosses the line.