NEW YORK - Novak Djokovic tipped his hat to Serena and Venus Williams ahead of their 27th career meeting but admitted Sunday he would find it impossible to play his own brother. 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. "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."