Wimbledon 2015: Serena Williams must beat Venus for history
Younger Williams has her in-form sister blocking route to first calendar slam
Elder sister Venus will directly block Serena Williams' calendar grand slam path today in a Wimbledon clash that is both essential viewing and impossible to watch.
Former Wimbledon champion Lindsay Davenport believes the inseparable siblings' first meeting in six years will have even the aficionados squirming in their SW19 seats.
Serena Williams admitted British star Heather Watson "should have won" their third-round battle, while also tipping Venus as the more in-form of the two five-time Wimbledon champions, whose 26th meeting launches Wimbledon's second week.
The Williams sisters have so few records left to break, but 33-year-old Serena "loves the position she's in" chasing unchartered territory by swiping all four majors in one season, according to fellow American Davenport.
"I think it's so difficult, I'm sure when they saw the draw they were both like 'ugh'," Davenport explained, 17 years on from the sisters' first meeting.
"Serena's playing for history; Venus isn't going to have that many more opportunities here. I'm sure it has to be so hard on them. They are obviously experienced at it, they have been dealing with it since they were 15 and 16 in age, but I'm sure they would rather see anybody else in the draw.
"I think people have a hard time watching them, because people think of them just always being together, even though they are both just amazing champions in their own right.
"It's a match you have to watch, but you almost don't want to at the same time.
"And I think again it's hard, everyone wants to see if Serena can complete this calendar year grand slam, there's a lot riding on it for her.
"But everyone loves Venus. So I think it's just a lot of mixed emotions seeing them play each other - especially in the round of 16.
"It would be one thing if it was a grand slam semi or a grand slam final, but fourth round just seems entirely too early for two five-time champions to play."
British number one Watson squandered two match points and one of the great Wimbledon upsets as Williams recovered her poise to set up this meeting with Venus.
The elder Williams eased past Aleksandra Krunic in straight sets, leaving Serena revealing practice-court frustrations over the 35-year-old's fine form.
Serena still boasts 20 grand slam titles to Venus' seven though, leaving 1999 Wimbledon champion Davenport tipping the 33-year-old to continue to home in on that elusive calendar grand slam.
"I think Serena loves the position she's in, playing for something she's never achieved before, going for the calendar slam," said Davenport.
Serena Williams has already bludgeoned her way to this year's Australian and French Open titles, and now has the season's third major title in her sights. "It's an exciting place for her to be and I think she's loving where she is right now," said Davenport.
"I think you can tell when Serena can play well, it almost always is when she's in a good place off the court, and in her personal life and a lot of times she brings some of the issues going on onto the court.
"Serena wants to go down as the greatest player of all time.
"Against Watson you saw some of the cracks and the pressure she's putting herself under to do something she's never done before in tennis."
Fifth seed Caroline Wozniacki must topple Spain's Garbine Muguruza to reach her first Wimbledon quarter-final in a packed fourth-round line-up.
Maria Sharapova takes on Zarina Diyas, while fresh from dispatching defending champion Petra Kvitova, Jelena Jankovic meets Agnieska Radwanska.
Eastbourne winner Belinda Bencic will face Victoria Azarenka, with Madison Keys up against Olga Govortsova, Timea Bacsinszky taking on Monica Niculescu and Lucie Safarova paired with Coco Vandeweghe.
Five previous meetings of tennis' sibling rivals at Wimbledon
The world-beating Williams sisters go head-to-head today in a Centre Court family battle for supremacy. Here we look at their five past meetings at Wimbledon.
2000, semi-finals, Venus beat Serena 6-2 7-6 (7/3): Twenty-year-old Venus and 18-year-old Serena slugged it out for a first Wimbledon final experience, but this was anti-climactic, not the classic many were expecting. Venus later said she was "appalled" by slurs that father Richard had fixed the result of the last-four clash.
2002, final, Serena beat Venus 7-6 (7/4) 6-3: More like it. The awesome resolve about Serena's victory silenced the grumbles of some of her vanquished opponents that the Williams family dominance was bad for tennis. Serena said afterwards: "I'm definitely good for the game. I'm really exciting, I smile a lot, I win a lot and I'm really sexy." This was the second leg of her 'Serena Slam', a non-calendar sweep of the majors which she completed with victory at the Australian Open the following January.
2003, final, Serena beat Venus 4-6 6-4 6-2: Venus defied a stomach strain and leg and hip injuries to ensure the final went ahead, but it was an altogether unsatisfying encounter. Venus clearly struggled physically through a contest described by John McEnroe as "unusual" and "a little inexplicable".
2008, final, Venus beat Serena 7-5 6-4: Thankfully this felt far more competitive than the sisters' encounter five years previously. Venus lifted the Venus Rosewater Dish for a fifth time - she has not raised it aloft since. One serve of 129mph - a Wimbledon women's speed record at the time - showed how hard she was going for this title.
2009, final, Serena beat Venus 7-6 (7/3) 6-2: Serena played a brilliant tie-break to end Venus' streak of consecutive sets won at Wimbledon at 34 - and from there onwards it was plain sailing. The champion was baffled afterwards that winning three slams in 10 months was not enough to displace Dinara Safina - who had none - as world number one. Statistics may have said one thing, but this left no doubt in most tennis observers' eyes about Serena's status as the world's premier female player.