World number two Rafael Nadal has admitted that his second group match against Roger Federer at the ATP World Tour Finals will be "difficult".
The Spaniard was speaking after he got his campaign off to a winning start against Mardy Fish this evening.
"It will be a really difficult match for me," Nadal told Sky Sports News. "[Federer's] playing fantastic and has won two titles recently.
"It is going to be a tough test for me."
The pair will meet on Tuesday.
