The Americans, who have both won five Wimbledon singles titles, share an incredible 27 Grand Slam titles.
Elder sister, Venus, 35, has steadily been recovering her form over the last 18 months after suffering from a debilitating autoimmune disease.
However, there has been no stopping Serena and the 33-year-old claimed her 20th Grand Slam title at the French Open last month to edge closer to a second 'Serena Slam' and her bid to win all four majors in a calendar year.
Tracy Austin, who won the US Open in 1979 and 1981: "It's incredible that two sisters, that are so close, have been vying with each other in majors for so long.
"You've got to give credit to their parents, Richard and Oracene, for keeping the family so tight and close. They have done an absolutely wonderful job.
"They made the two girls realise from an early age that every time they play it is just a tennis match."
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