Roger Federer has said that his serve out wide was key to defeating Novak Djokovic in the group stages of the ATP World Tour Finals.
The world number three qualified for the semi-finals with a 7-5 6-2 triumph against four-time champion Djokovic, who must beat Tomas Berdych on Thursday to guarantee his place in the last four.
After the match, the 34-year-old told Sky Sports: "The tactic was to mix it up. I have to against Djokovic.
"I hit it well out wide today. I knew it was an important serve to get Novak off the court."
Federer's victory was his 22nd over Djokovic in 43 meetings between the two players.
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