NEW YORK: Novak Djokovic is best known for his talent on a tennis court and cheeky sense of humor, but he has shown a more serious side to his nature at this year's US Open.

The tournament, because of its timing, will inevitably always be clouded by painful memories of the September 11, 2001 terrorist attacks on the World Trade Center in Manhattan which killed almost 3,000 people.

And it was with that in mind that the 22-year-old Serb, a finalist here in 2007 and the Australian Open champion the following year, invited children of 9/11 victims to share his players' box on the Arthur Ashe Stadium court.