Rafa Nadal and Novak Djokovic must vie for Diego Maradona support

Rafa Nadal and Novak Djokovic have enjoyed the support of Diego Maradona during their opening matches at the World Tour Finals but the Argentine soccer great will have split loyalties when the duo meet later today.

The top two players in Group A won their opening round-robin encounters and victory on day four of the season-ending championships will almost certainly secure them a place in this weekend's semi-finals.

French Open, Wimbledon and U.S. Open champion Nadal has a commanding 15-7 advantage in their head-to-heads but significantly on hardcourts, Djokovic has come out trumps in seven of their 11 meetings.

With this in mind, Nadal knew he would have to do better than his three-set tussle against Andy Roddick on Monday.

"Right now I have to play better. I have to play with a little bit more energy," said the world number one, who lost all six sets he played here last year.

"I have to play with a little bit more positive attitude."

Maradona has been a regular visitor this week to the O2 Arena on the Thames riverbank and his presence has energised Djokovic, who thumped Wimbledon runner-up Tomas Berdych on Monday.

"He's going to stay throughout the whole tournament. Hopefully he can be my lucky charm," Djokovic told reporters.

Asked about his tactics against Nadal, he added: "You have to find energy. You have to find focus to perform as best as you can."

Roddick and Berdych will be searching for their first wins this week when they meet in the afternoon session.