Serena and Venus Williams will meet for a place in the US Open last 16 on Friday. AFP Sport looks at six of the most memorable clashes between the American sisters whose rivalry stretches 20 years.
1998 Australian Open 2nd rd
Nowhere near a classic -- it was a described as 'sub-par' by one newspaper -- but noteworthy as it was the first meeting between the two sisters. Both appeared sporting beads in their hair. Venus, 17 at the time, triumphed 7-6 (7/4), 6-1 which was expected as she was ranked 16 in the world while 16-year-old Serena was down at 53.
2003 Australian Open final
One of the most dramatic finals between the sisters came in the heat of Melbourne, when Serena clinched the last Grand Slam title to elude her with a tension-filled 7-6 (7/4), 3-6, 6-4 victory.
It was only the sixth time a woman had held all four of Grand Slams at the same time, and the first since Steffi Graf in 1994.
Venus made unwanted history as the first woman to lose four straight Grand Slam finals.
2008 Wimbledon final
Venus clinched her fifth Wimbledon title with a 7-5, 6-4 win in one of the few really high-quality clashes between the pair at the All England Club.
Venus defied blustery conditions to become one of only four players to lift the trophy five times or more in the modern era.
She sealed the historic victory when Serena fired a backhand wide, but her celebrations were muted as she hugged her sister at the net.
2009 Dubai semi-final
Venus beat top ranked Serena 6-1, 2-6, 7-6 (7/3) to take a 10-9 lead in the sibling rivalry.
It was Venus's fifth win in their last seven meetings but Serena earned plenty of plaudits as well for defying the pain of a knee injury that had forced her to retire from a tournament the previous week.
It was only the second time a set had gone all the way to a tie-break in their 19 career meetings to that point.
2009 Wimbledon final
Serena ended Venus's two-year reign as Wimbledon champion with a 7-6 (7/3), 6-2 victory that secured her a third All England Club crown.
It was the fourth time the sisters had met in a Wimbledon final, with Serena, who now held the Wimbledon, Australian and US Open crowns, winning three of those showpiece showdowns.
She regained the Wimbledon title she won in 2002 and 2003, and shattered five-time champion Venus's hopes of lifting the trophy for a third successive year.
2017 Australian Open final
Serena secured her seventh Australian Open and an Open-era record 23rd Grand Slam singles title with a 6-4, 6-4 victory, just one shy of Australian Margaret Court's all-time mark of 24.
It may not have been a classic clash but the enormity of the achievement only became clearer later when it was revealed that Serena was already pregnant with daughter Olympia.
"The last time we met at a Grand Slam, it was two against one," joked Venus after making the third round of the US Open on Wednesday.