If it was anyone else, they would almost certainly have quit tennis by now.

They would be lying on a beach somewhere, enjoying being a seven-time grand slam-winning champion and a multi-millionaire, as well as having cemented the legacy of being one half of the world's most famous sporting sisters. But for Venus Williams, quitting was never an option.

This week at the Australian Open, the 34 year-old has had her most successful grand slam tournament for over four years by reaching the quarter finals. This week's success is particularly impressive when you consider the challenges that the elder Williams sister has had to face throughout the last four years.

In 2011, the American revealed that she had Sjogren's syndrome, an auto-immune disease which causes chronic fatigue and muscle soreness. In the past few years, Williams has played matches that she clearly should not have played, looking gaunt, weak and overpowered by her opponent and losing to players who, at her peak, would not have got close to her.

Sjogren's syndrome is incurable but it is a condition that Williams has, slowly but surely, learned how best to cope with. She has altered her diet and put more emphasis on rest and recovery but it remains unlikely that Williams will ever return to the form which took her to seven Grand Slam titles, including five Wimbledon crowns.

For most people, such an obstacle would have been too much to battle, particularly when one considers how demanding the physical aspect of being an elite women's tennis player is these days. But not for Williams. After dropping as low as 134 in the world, she has steadily fought her way back up the rankings and was seeded 18th at the Australian Open.

Her run to the quarter finals was truly incredible, with her most impressive victory this fortnight being over sixth-seeded Agnieszka Radwanska of Poland in the fourth round. This Australian Open has been Williams best grand slam performance since Wimbledon in 2010 when she reached the semi-finals. And her recent improvement and consistency of results on tour is beginning to bear fruit - in Monday's updated ranking list, Williams is projected to rise to number 11.

But why does she continue to plug away? Williams is some considerable distance from the form that made her, at one point, an utterly dominant force on the tennis court and her legacy as one of the most successful female tennis players of all time is safe - only Martina Navratilova and Steffi Graf have won more Wimbledon titles than Williams in the open era. Together with her sister, the pair have changed the way women's tennis is played forever.

So, it seems that Williams has little to gain by continuing to trudge on in a seemingly futile attempt to add to her tally of grand slams. But Williams is a fighter. Nothing has ever come easy to either of the sisters; ever since their father decided when they were children that they were to become tennis players, they have had to fight against the tide to get to where they are now.

When Williams began playing the sport in Compton, Los Angeles, the area in which they trained was a notorious gang-land and drug-dealing area and the court they played on was cracked, with pot-holes and a ripped net. Even in these early days, Venus was told by her father that she would be a champion.

That Williams overcame all of this adversity to become the best woman's tennis player in the world is, perhaps, what has given her the belief that she can overcome Sjogren's syndrome. Earlier this week, she opened up somewhat on why she continues to play tennis; "For me to be out here now, it's to prove to myself that I won't let anything defeat me", she said. And it is this attitude which has allowed her to excel throughout her career; whatever challenges she faces, nothing seems insurmountable.

Without their tough upbringing, would either of the Williams sisters have been quite so prolific as they have turned out to be? It is, of course, impossible to say definitively, but it seems likely that their mental resilience is the thing that sets them apart from their peers. Much of their success is credited to their physicality yet their mental toughness is equally, if not more important.

Williams cites one person in particular who has been her inspiration; her sister, Serena, calling her "the ultimate champion". "She's always been someone that anyone can learn from", Williams said. "The way she faces her life, the way she is fearless on the court- I don't think I could have done the things I've done without her."

Serena continues to reign supreme in the women's game - tomorrow, she aims to capture her 19th grand slam title. It is questionable as to whether either of the Williams sisters, but particularly Venus, has been granted the credit they deserve for their achievements.

The debate over why, is seemingly never-ending. Some have ventured that it is related to their sex, their race, or the fact that the sisters don't conform to the standards of the great female champions of old, where grace and finesse were seen as the finest traits.

In spite of her resurgence this week, it would take a very brave pundit to suggest that Venus will add to her grand slam haul. In many ways that is irrelevant now. Because her never-say-die attitude, which has been so obviously on display in Melbourne this last fortnight, has served to win her new fans and even more respect. Her legacy has a new, shimmering gloss.