They were still teenagers. It was the second round of the 1999 Australian Open, and Venus won in two sets.
"I totally remember," Venus Williams said Friday at the US$2.44 million Rogers Cup. "We had a tough match back then.
"That's the trend that started and it hasn't changed. I suspect this will be another tough match."
The 34-year-old Venus and the 32-year-old Serena have their 26th career meeting in the semifinals of the hardcourt event at Uniprix Stadium on Saturday.
Serena, ranked first in the world, is 15-10 against her 26th-ranked sister, although one was a walkover when her sister withdrew. She has won the last five head-to-head matches.
"I have to play well," added Venus Williams, whose last win over Serena was at Dubai in 2009. "There's no secret or science to it.
"I think anyone who has got any wins against her, they've pretty much played the match of their life. Granted, I'd like to imagine that I won't have to play the match of my life. That's tough to do. But I know I need to play well."
Both rallied from one-set deficits to win their quarter-final matches.
Serena Williams defeated Denmark's Caroline Wozniacki 4-6, 7-5, 7-5 while Venus Williams dispatched 14th-seeded Carla Suarez Navarro 4-6, 6-2, 6-3.
"I definitely don't like playing her," Serena Williams said of her sister. "I think I've lost to her more than anyone on the tour, so it's definitely not a fun match.
"She's tough. She has a great serve. She runs every ball down. She has a great backhand. She hits winners off the forehand. She does everything well so it's not an ideal matchup for anyone, to be honest."
In another quarter-final, Ekaterina Makarova ended the Cinderella run of American qualifier Coco Vanderweghe with a 6-1, 4-6, 6-1 victory. Vanderweghe had knocked off two former world No. 1's en route to the quarter-finals.
Makarova will play third-seeded Agnieszka Radwanska of Poland, who got a 6-2, 6-2 win over Victoria Azarenka, a Belarusian whose movement was troubled by a heavily taped right knee.
The sisters will meet for the first time since Serena Williams cruised to a 6-1, 6-1 win on clay at Charleston, S.C., in 2003.
Between them, the Williams sisters have won 24 grand slam tournaments, with Serena taking 17 and Venus seven.
They have also been credited with introducing more of a power game to women's tennis, and of inspiring players from minority communities to take up the sport.
"We've had such a great impact," said Serena Williams. "I think having been pretty much one of the first African-Americans to do well, then to see all the African-Americans, even some Canadians here playing really well, it makes me feel really good.
"I'm really excited to see that. We just had an opportunity to do something really special. Sometimes I get chill bumps when I think about it because we're just so normal and so down-to-earth. We just feel like we want to help everyone to do the best that they can."
Serena Williams, the defending champion, extended her Rogers Cup win streak to 14 matches after victories at the 2011 and 2013 events in Toronto. She skipped the 2012 tournament.
Wozniacki, coming off a win at Istanbul, ended an eight-match streak.
The match that lasted two hours 41 minutes saw Wozniacki take control in the first set and build a 2-0 lead in the second before Williams started hitting the lines.
"I was making a lot of unforced errors in general," said Williams. "But I just thought: 'Just keep fighting. Hopefully I can just hold on and hold on, try to break, keep going for each shot.' "
She broke service for a 3-2 first-set lead and fought off two break points to close the set.
The third set featured five service breaks, the last giving Williams the win on her third match point.
Williams is now 7-1 all-time against Wozniacki. Williams won at Stanford last week and has won seven straight matches.
But she knew beating Wozniacki wouldn't be easy.
"Especially when you play someone like Caroline or other opponents, you know your match might be a little longer than normal," said Williams. "You go in there thinking: 'This is not going to be routine.' You're ready."
Wozniacki was happy with her tournament despite the defeat.
"It was really small things that did it out there today," she said. "She came up with quite a few big serves when she needed to.
"It could have gone both ways. I had a break in the second set. Didn't manage to hold on. But I gave it everything I had. It just wasn't quite enough."
Venus also needed time to find a rhythm against 14th-seeded Suarez Navarro. She has gone three sets in three of her four matches, including her third-round upset of sixth-seeded Angelique Kerber.
"What's happening well for me is just competing," said Venus Williams. "I think my movement has been very helpful to me, as well, covering a lot of court.
"I've been playing some stellar opponents, so it hasn't been easy."