Oracene Price, the mother of Serena and Venus Williams, understands the dilemma facing the older of her two daughters when they meet in the quarter-finals of the US Open tonight.
Venus last won this title 14 years ago and has not made the semi-finals since 2010. At 35 this could be one of her last opportunities to win an eighth Grand Slam title.
Victory, however, would deny her sister the chance to make history by winning a calendar-year Grand Slam.
Price, who said she did not plan to attend the 27th meeting between the sisters, said: "I know it's going to be hard, because I know Venus wants Serena to get it."
Venus believes she is playing well enough to win her third US Open title. Although she was taken to three sets by Monica Puig and Irina Falconi in her first two matches, the five-times Wimbledon champion beat Belinda Bencic, the No 12 seed and one of the game's most exciting young talents, in straight sets in the third round and needed only 50 minutes to brush aside Anett Kontaveit in the fourth.
Serena, nevertheless, provides a challenge of a different dimension. Although Venus won more of their early meetings, Serena has won 13 of their last 19, including the most recent, at Wimbledon this summer.
Venus, who has continued to excel despite being diagnosed four years ago with Sjogren's syndrome, an incurable auto-immune condition that causes fatigue and joint pain, has not beaten her sister in a Grand Slam tournament since the 2008 Wimbledon final.
Venus, who reached the final on her debut at this tournament 18 years ago, recalled: "I was always so tall and she was really small at the time. So it wasn't really a fair match." While Serena eventually proved to be the more successful of the siblings -she has won a total of 21 Grand Slam singles titles while Venus has won seven - the world No 1 said she still regarded her sister, who is currently the world No 23, as her most dangerous opponent.
"She's the only player in the draw I don't want to play, not only because she's my sister, but because for me she's the best player," Serena said. "She's beaten me so many times. I've taken a lot of losses off 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.
"We have a very similar game. She's fast, I'm fast. She hits hard, I hit hard. She serves big, I serve big. We had the same coach for a long time. It's like playing a mirror. I have to be really ready."
Venus was asked what the biggest challenge was in facing her sister: "Well, obviously that she's so good," Venus said. "What else can you do except try to win the point and hope she doesn't hit an ace?"
There was a time when Serena hated playing her sister, but she said that had changed. "I think it's more fun than it used to be," she said. "We really relish the opportunity. We're both happy still to be involved in getting so far. And it's still super-intense. She's doing well and she wants to win this. So do I.
"It's been a great experience. I feel like Venus and I have proven that you can be friends and you can be sisters, you can be enemies on the court and friends and sisters off the court."
Simona Halep, the world No 2, survived a marathon against Sabine Lisicki, winning 6-7, 7-5, 6-2 and in the quarter-finals Halep will face Victoria Azarenka, who beat Varvara Lepchenko 6-3, 6-4.