LONDON: Serena Williams has called on fans to support her sister Venus when the two five-time Wimbledon champions meet in an emotional fourth round clash on Monday.

The sisters, who have 27 Grand Slam titles between them, will be on opposing sides of the net for the sixth time at Wimbledon and for the 26th time in their careers.

Venus, 35, thrashed Aleksandra Krunic to reach the last 16, while Serena, 33, had to survive an almighty scare against Britain's Heather Watson before guaranteeing the first all-Williams tie at Wimbledon since 2009.

Since Serena won that final clash six years ago, the sisters' paths have diverged with the younger sibling putting herself among the all-time greats by reaching 20 Grand Slam titles.

In contrast, Venus has slumped down the rankings as she fights a constant battle with auto-immune disease Sjogren's syndrome that often leaves her exhausted.

As a result, Serena believes Venus's struggles deserve the sympathy of the All England Club crowd if they have to pick sides.

“I expect more people to be rooting for Venus,” said Serena. “I would be rooting for Venus. She's been through so much. She's had a wonderful story.

“She's been so inspiring to me; she's just an incredible individual.

“She's just so amazing. She's been so inspiring to a lot of people with the same things that she goes through, too.”

Venus returned the compliment, claiming she would support Serena if she were a neutral bystander on Monday.

“That's interesting, I'll probably be cheering for her,” Venus said when told of Serena's desire for the crowd to support her.

“I haven't given any thought to the crowd really. There's no such thing as giving up for me in my life.

“But also for her, she's been through so much.

“You see players that continue to talk about injuries that are like 9,000 years old. She never complained once. You have to give her credit for that. ”

After four Wimbledon finals and a semi-final between the sisters, this will be their earliest meeting on the lawns of south-west London.

On the surface, world number one Serena, chasing her fourth successive Grand Slam crown and 21st of her career, is the clear favourite to secure her 15th career victory against 16th seed Venus.

But, regardless of the result, Serena says seven-time major winner Venus has already left a great legacy as a “pioneer” for both the sport and women's rights.

“Where do I start?” asked Serena, when asked to chart Venus' tennis legacy.

“She's done so much for this sport.

“It's just been amazing for me to follow in her footsteps in terms of getting equal prize money here, just stepping up to the plate and being that person.”

Grand Slam duels have never sat easy with the Williams family bonds, and, with only their third meeting in six years just days away, Serena admitted time has not eased those conflicting emotions.

“It's unfortunate that it's so soon, but we're going to do the best that we can,” she said.

“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.”

But she added: “I just don't know how I'm going to do, to be honest. I'm playing the toughest player I've played in women's tennis.”

Federer, Murray target Wimbledon last 16

Seven-time champion Roger Federer and 2013 winner Andy Murray look to reach the Wimbledon fourth round on Saturday.

Federer bids to extend his 18-match winning streak against Australian opponents at Grand Slams when he takes on big-serving Sam Groth in the opening match on Centre Court.

The 33-year-old Swiss beat Groth, the world number 69, in straight sets at last year's US Open.

Groth is bidding to make the fourth round at a major for the first time and join compatriot Nick Kyrgios in the second week.

Murray, the third seed, has a 6-1 career lead over Italian veteran Andreas Seppi ahead of their showdown.

Seppi knocked Federer out of the Australian Open in January but his only win over the British star came back in 2006 in their first meeting.

The 31-year-old Seppi has reached the fourth round on four occasions — at the 2012 French Open, 2013 Australian Open and Wimbledon and this year's Australian Open.

Sixth seeded Czech Tomas Berdych faces Spain's Pablo Andujar.

In the women's draw, defending champion Petra Kvitova, who has only dropped three games, faces Serb 28th seed Jelena Jankovic.

Kvitova has a 4-2 lead over Jankovic but the two have never met at a Grand Slam. Fifth seed Caroline Wozniacki faces Camila Giorgi trailing 2-1 in their career meetings. Giorgi defeated the Dane at the 2013 US Open.