The world No. 2's face had a pained look during the match when he netted with most of the court open and his opponent -- having all but given the point up -- bent over and facing the other way at the net.
But Federer saw the funny side later: "It was a big distraction, I'll tell you that, because [Sock's posterior] was very big. That's what I should have aimed for. That target was bigger than the down-the-line court that I had."
When Sock was asked if he was surprised Federer had missed that shot, the American responded that it wasn't the first time he had turned his back like that. "I did it more for fun," he said in his post-match news conference. "Probably do it three times a year. No, it's not a normal tactic."
Nadal on course for Monday match
Rafael Nadal said "see you tomorrow on court" as he signed off from an address to the Centre Court crowd after receiving his year-end world No. 1 trophy at the end of the Federer-Sock match.
It was hardly a categorical statement of his fitness, and he will still check on the problematic right knee Monday before going on to face David Goffin in his first match of the round robin.
But the signs earlier in the day were good as his coach and uncle, Toni Nadal, gave something verging on an update to ESPN:
Coach Toni Nadal was in enigmatic form when I chatted to him today. Me: Will Rafa be fit to play the @ATPWorldTour finals Monday? TN: "Hopefully"; how confident are you? "We know but I don't want to tell you." @RafaelNadal without any strapping, looked in good shape though. pic.twitter.com/NM4nArqVKW— Leo Spall (@LeoSpallESPN) November 12, 2017
Murray still on the mend
Rafael Nadal hasn't been the only injured member of the Big Four getting back to full fitness at the ATP Finals in London.
Last year's world No. 1, Andy Murray, who is nursing a hip problem, was also practising at the O2 Arena this weekend; he was moving better, too, than he had in Glasgow last week for his exhibition match with Federer.
Early birds catch doubles magic
There was a bit of a late rush from fans to get into the O2 Arena in time for the start of the opening session of the ATP Finals at midday.
Maybe it was just a weekend thing, or the fact that the first match on was doubles, but the crowd -- eventually filling the stands around the court -- couldn't complain about a lack of entertainment.