What is making Wimbledon's Centre Court so slippery?
World number one Novak Djokovic said he did not recall "falling this many times on court" after several tumbles in his opener against Britain's Jack Draper on Monday, while Andy Murray says it is "extremely slippy out there".
Federer suggested it was "more slippery under the roof", which was on for much of Tuesday's play and some of Monday's while rain fell at SW19.
"You do have to move very, very carefully out there," Federer said.
"I do think it feels a tad more slippery maybe under the roof.