IT IS the head to head that should have been the Navratilova versus Evert of its generation: the clash of the two best players in women's tennis, with no holds barred and no quarter given.

Instead, the on-court meetings of Venus and Serena Williams have at times had all the confrontational power of two octogenarians having a natter over a cup of tea.

The sisters, who are first up on Centre Court on the day when the last 16 men and women all play, have met 25 times so far in their careers. Four of those matches have been in the Wimbledon final, with Serena winning in three. The 2008 final, won by Venus, was a close and captivating contest, but others have been anticlimactic in the extreme.

At times this was because one was carrying an injury; at others because, having ensured the title would stay in the family, they did not always look too concerned about which of them would triumph. Although played out on the biggest stage in the sport before the world's media, Williams versus Williams has often been a strictly domestic affair with no wider resonance.

In recent years the match-up has even lost the potential it once had to showcase two world-class athletes at the height of their powers, as the gulf in achievement has grown between the two. Venus is 35 now, her best years some way behind her, while Serena, though only a year and a bit younger, remains the greatest power in the game. It is because of that disparity that they are meeting so early in the tournament: Serena is the top seed, Venus the No 16.

Nonetheless, it would be wrong to conclude from that difference in rankings that there can be only one winner today. Venus has looked rejuvenated so far this fortnight - above all in her first round, when she double-bagelled Madison Brengle - while Serena has been below her best. The world No 1 conceded five games in each of her opening matches, and then in the third round against Heather Watson came within a couple of points of defeat.

"I honestly didn't think I was going to win," Serena said after that match against the British No 1. "How I pulled through, I really don't know."

But she did pull through, thus remaining on course to complete the so-called Serena Slam - holding all four majors at the same time, though without having won them all in the same year. After the match against Watson, however, she refused to discuss that goal, insisting she could look no further forward than the next round.

"It's unfortunate that it's so soon," she said of her meeting with Venus. "But we're going to do the best that we can.

"I mean, she's my sister today, she's my sister next week, she's my sister next year. I think that's a little more important than a match.

"We'll leave everything out on the court. When it's done, we'll go back to regular life.

"I think it will be a really good match. She's playing so well. I'm practising next to her every day and I'm in awe of how she's doing. It's a little frustrating, because I know I have to play her. I just don't know how I'm going to do, to be honest."

For her part, Venus said that one of Serena's greatest strengths, as her win over Watson demonstrated, was her ability to emerge victorious even when playing below her best form. "I think no matter how Serena's playing, she knows how to win," the elder sister said. "I think I'm playing very well, actually. Just have to keep that up.

"We've been facing each other a long time, so we both have a lot of respect for each other's game. We both play quite well. We both know we have to play well against each other."

If they both play well, Serena will win. Even if Venus plays better, Serena should still win, because of that ability to hang on when playing badly and still seize the moment.

If Venus upsets the odds and wins, the women's draw, already thrown wide open by the defeat on Saturday of defending champion Petra Kvitova, will become even less predictable. Maria Sharapova, the 2004 winner, would then be the highest remaining seed at No 4, presuming she wins her own fourth-round match against the unseeded Zarina Diyas from Kazakhstan.

Should Sharapova go through, her quarter-final opponent will be either Lucie Safarova, the No 6 seed, or Coco Vandeweghe, the unseeded American who knocked out Sam Stosur in the last round. The winning Williams will have a last-eight match against either Victoria Azarenka, the former world No 1 who is steadily heading back towards the top after a string of injuries, or Belinda Bencic, the 18-year-old from Switzerland who won her first WTA title at Eastbourne last month.

All those possible matches are in the top half of the draw. In the bottom, another former world No 1, Jelena Jankovic, is on a high after knocking out Kvitova and should be too powerful for Agnieszka Radwanska, the No 13 seed from Poland.

Madison Keys, the 21st seed from the United States, has made the most of a favourable draw. She should beat her fourth unseeded player in a row when she takes on Olga Govortsova, at 122nd in the world the only survivor who is outside the top 50.

Like Jankovic, No 5 seed Caroline Wozniacki is coming into form at the right time - although she will need to keep improving if she is to reach the quarter-finals. Her last-16 opponent, 20th seed Garbine Muguruza, knocked out the No 10, Angelique Kerber, in the last round.

Timea Bacsinzsky, the 15th seed from Switzerland, is only just beginning to realise the promise she showed as a junior. The 26-year-old is up against the unseeded Romanian Monica Niculescu, who has won all three of their tour meetings so far.