Though Nadal has finished the year at the top of the world rankings, Federer is the odds-on favourite to end his incredible year with a win in London.
But how good are they at deciphering cockney rhyming slang?
Sock will hope to defeat Zverev in the final round-robin meeting in the Boris Becker group in a match which could decide who progresses to the semi-finals while Cilic will face Federer hoping to finish his ATP Finals campaign with his first victory of the week. "I think I first qualified for this tournament in Shanghai in 2002 when I was 21, so I'm very excited for Alex - he has a very bright future ahead of him".
Federer didn't panic, though, and served his way out of trouble, with Zverev missing an early opportunity to snatch the initiative.
It was the American world No 9, the Paris Masters champion, who started the quickest breaking his Croatian opponent in the opening game of the match but he was broken back immediately.
He hit a powerful forehand victor to let his intentions known early on and set up three break points in the opening game.
While others might have been surprised at Sock becoming the first American to win at the elite season-ending event since Andy Roddick in 2007, the man himself wasn't.
Sock twice served well under pressure to stay in the match and despite falling a mini-break behind in the tiebreak recovered to win five unanswered points to claim victory.
The Swiss went on a run across the end of the first set and beginning of the second, winning 16 of 19 points as Zverev crumbled under the lights and noise of 17,000 partisan fans.
"Definitely disappointing in both matches in that third set, being in a good position, putting myself in a good position", he said.
Though Federer radiated frustration at his own performance, he kept his focus and broke for a 2-1 lead in the decider. He eventually took the chance, his sixth of the match, running down another loose Federer drop shot.
Federer only landed 43 percent of first serves in the second set and it finally cost him in the 12th game, when Zverev capitalised on his third set point to force a decider. But there were three of them on this occasion and Federer could only save two of them.
Zverev looked as though he had put the disappointment behind him when he jumped out to 40-0 in his next service game, but once again, the forehand errors came to ruin his day and gave Federer five straight points for the double break.