![Serbia's Novak Djokovic returns the ball to Switzerland's Roger Federer.](https://thenational-the-national-prod.cdn.arcpublishing.com/resizer/v2/6M4YX2TYGWKNAHZLYK7FQWLRVE.jpg?smart=true&auth=fd73413f877e42c334123c0d8dc73114aa6c578685166dc6c0acdb138077c372&width=400&height=225)
Serbia's Novak Djokovic returns the ball to Switzerland's Roger Federer.
Serbia's Novak Djokovic returns the ball to Switzerland's Roger Federer.
Djokovic up for 'ultimate challenge' with Nadal
History in the line as Serb meets Spaniard for fourth time in a year, for a rare Grand Slam and beat Federer's 27 successive wins in majors.