Novak Djokovic "got his karma back" for inadvertently striking Roger Federer when Jack Sock did the same to him in the Laver Cup.

Djokovic and Federer teamed up on Friday as they took on Sock and Kevin Anderson in the doubles, suffering Team Europe's only defeat of day one with a 6-7 (5-7) 6-3 10-6 loss.

Their lack of familiarity as a pair was perhaps best evidenced when Djokovic struck Federer on the back with a forehand, prompting an immediate apology from the Wimbledon and US Open champion.

"I apologized right away," Djokovic said. "I got my karma back when Sock hit me right in the heart!"

"It was great to play with Roger, it was great fun. We talked a lot of strategies on the bench. We have always had plenty of respect for each other, now that will be strengthened. But this kind of quality time is like no other."

Federer has previously played with Rafael Nadal in doubles and added: "Rafa and Nole are very different players, a lot of changes, opponents were very different too. Excitement was similar for me, to team up with someone of Nole's calibre was a treat."

"You learn a lot from these matches. When you play doubles it's different, it's like speaking a different language, using a different side of your brain. And in a match like today you may go through games when you get no rhythm."