Novak Djokovic has revealed that he has been watching the performances of Rafael Nadal during the ATP World Tour Finals in London.

After a disappointing year, Nadal has impressed at the O2 Arena, winning his three group matches against Stanislas Wawrinka, Andy Murray and David Ferrer to earn a semi-final match with Djokovic.

However, the world number one has insisted that he will not be surprised by the game of Nadal, who he says he has been watching during the tournament.

The 28-year-old is quoted by BBC Sport as saying: "It's a fact that he's been playing better and he's been raising his level ever since the US Open.

"The first couple of matches he played here against Wawrinka and Murray showed that he feels more confident, shows that he's starting to miss less, serve efficiently, use his forehand much better. I know that - I've been watching."

This afternoon's clash will represent the 46th time that the two players have met, with Nadal edging the head-to-head record with 23 wins compared to 22 for Djokovic, although it has been over two years since the Spaniard prevailed on a hard court.