OF all the potential obstacles preventing Serena Williams from making history by winning the calendar year grand slam, surely the only one she didn’t consider was that her sister could be the one to stop her.

And yet, irony of ironies, Venus Williams will be the one standing in her little sister’s path when the two meet in the quarter-finals of the US Open in New York today.

It’s hard to imagine a worse scenario for either; on the one hand, world No.1 Serena knows how good Venus is and that she can still, on her day, be a massive handful.

“For me, she is the only player in the draw I don't want to play,” Serena said. “Not only because she's my sister, but for me she's the best player.

“She's beaten me so many times. I've taken a lot of losses off of her – more than anybody. She's a player that knows how to win, knows how to beat me, and knows my weaknesses better than anyone. So it's not an easy match at all. Hopefully things will go right. We'll just let it slide off our shoulders. We don't take it personal.”

On the other, imagine the guilt she will feel if Venus is the one to stop Serena from becoming only the fourth woman, and the first since Steffi Graf in 1988, to win all four grand slams in a calendar year.

These are two women who not only changed the way the game is played, they lived in each other’s pockets for the best part of 30 years, first at home with their parents and later, sharing a house in Florida.

Venus won their first meeting, 17 years ago at the Australian Open but Serena leads 17-11, matches that have ranged between the awkward and the brilliant.

At 33, two years younger than Venus, Serena is only one grand slam title between Graf’s Open era record of 22 and while she is unlikely to get another chance to win the calendar year slam, Margaret Court’s all-time record of 24 is within sight.

Serena maintains she is not feeling any pressure, that winning this tournament “will not make or break my career” but the irony of the situation is not lost on Venus.

“I don't think anyone wants to be a spoiler,” she said. “I think people love to see history being made. No one is out to be a spoiler but, at the same time, you're focused on winning your match.”

It is four years since Venus was diagnosed with Sjogren’s syndrome, an incurable autoimmune disease that causes joint pain and chronic fatigue.

Though she doesn’t know how she’ll feel from day to day, the 35-year-old has confounded many medical experts who said she would not be able to play at the same level.

Yesterday, they practised side by side on adjacent courts. Whatever happens today, the two will shake hands and go back to being sisters, before rivals.