There is very little about the careers of Serena and Venus Williams that is not remarkable and even as they hit their mid-30s they remain huge forces in the tennis world.

That world No 1 Serena will tomorrow play for a place in the quarter-finals of the Australian Open is no surprise - she owns 17 Grand Slam titles and has won five times in Melbourne.

But the fact she is joined by her elder sister, Venus, is a nice bonus, with the former world No 1 making it through to the last 16 of a Grand Slam for the first time since she managed it at Wimbledon in 2011.

That was just before she announced she was suffering from Sjogren's syndrome, an auto-immune disease that causes chronic fatigue and muscle soreness.

Now 34, Venus Williams does not know how she is going to feel from one day to the next, hardly the ideal situation for someone hoping to compete at the business end of Grand Slam events.

But though Sjogren's has no cure, it seems Venus is learning to cope with the illness to a level where she is back winning titles, as she proved with victory in Auckland the week before Melbourne.

Yesterday, Williams managed a 4-6, 7-6, 6-1 victory over Italy's Camila Giorgi, having trailed 4-2 and 40-0 at one stage in the second set.

It is a result that will push her to the verge of the world's top 10, even if she fails to get past sixth seed Aga Radwanska in the next round.

"This little cat still has a few tricks," she said, before elaborating on the day to day struggle of coping with such an illness. "Definitely the years go by fast. It's definitely been a lot of work and a lot of learning and a lot of perseverance. It will continue to be that for me. Just have to come to terms with it.

"I just do my best every single day. That's pretty much the way it goes. I think just not being afraid is the most important thing."

Having won seven Grand Slam titles, Venus wants more.

"I always believe in my ability but sometimes you don't get results for whatever reason," she said. "Could be many. I think as long as I know that I've got it in me, that's it. I've won big. It's not like I haven't done it before. I like to win titles, whether it's a smaller event or a big event. That's what I play for.

"So it's great to be in the second week. But is fourth round my goal when I come to these tournaments? No."

While Venus rolls back the years, top seed Serena struggled yesterday but showed her fighting qualities to battle past Elina Svitolina of Ukraine 4-6, 6-2, 6-0.

It has not been vintage Serena, yet, but she's into week two and now just four more wins from moving ahead of Martina Navratilova and Chris Evert in the all-time list of Grand Slam champions with 19.

Evert once wrote an open letter to Serena, asking her to focus on her tennis rather than her off-court interests and the former world No 1 said she has no doubt that Serena will win No 19 sooner rather than later.

"I don't know if it's here but no doubt in my mind she can get to 19," Evert said. "I'm surprised it took her [so] long to get to 18 - I think she put a lot of pressure on herself - but it wouldn't surprise me if she won one or two this year, so I think that's a pretty safe bet. I'm pretty much 95% sure she'll make 19. Good for her, she's been through a lot."

Serena's opponent tomorrow will be Garbine Muguruza, the Spaniard who beat her at the French Open last summer. Describing it as her "best loss of last year", the American said she will be ready.

Two-time former champion Victoria Azarenka continued her resurgence as she beat Barbora Zahlavova Strycova 6-4, 6-4 but Wimbledon champion Petra Kvitova was upset 6-4, 7-5 by American Madison Keys, who played the match of her life.