The world number one has two tennis-playing younger brothers — 24-year-old Marko and Djorde, who’s 20.
Both play on the sport’s lower-level tours, which suits Djokovic just fine.
That’s a luxury which neither Serena nor Venus can afford and they clash again on Tuesday for a place in the US Open semi-finals with Serena chasing a calendar Grand Slam.
“The first thing that comes to my mind is how would I feel to play my brother, and I don’t think that would be possible, honestly,” said Djokovic after reaching his 26th Grand Slam quarter-final Sunday in New York.
“In a way I admire what they’re doing for so many years to play against each other on such a big stage. It’s strange. They are sisters. They live together. They grew up together playing tennis.
“But I don’t know if I’ll be able to play my brother. It’s not possible. I would not be able to handle it on the court.”