Tournament favourite and six-time victor Roger Federer has meanwhile enjoyed a far more tranquil build-up having missed the Paris Masters after winning the Swiss Indoors, to ensure he was fully fit for the London showcase. "But I didn't expect him to be this good yet".
Federer, in London for the ATP Finals, starting on Sunday, said he had enjoyed watching the matches on TV but urged caution.
Heading into the final tournament of the year - the ATP Finals - the duo are considered the favourites to add one more title to their collection.
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.
The 20-year-old German, ranked three in the world, was hot and cold against Croatia's Marin Cilic but, when it mattered most, he produced his best tennis to win 6-4 3-6 6-4.
"I feel fine", said the 36-year-old.
More errors had crept into the Zverev game and although the youngster got the help of Hawk-eye to deny Cilic a set point, the Croat soon served out to square the contest. "I'm here to try my best to give me a chance".
Federer won the final two points on his serve to go to 1-0 in the round robin.
Cilic was perhaps still thinking about a missed opportunity when he blew a 40-15 lead in the next game, losing it thanks to another excellent lob from Zverev - one Cilic did get a racquet to but was unable to control.
There was a moment of levity towards the end of the first set when Sock, who had messed up an attempted drop shot, showed his posterior as a supposed target for Federer, who proceeded to hit the net.
Federer coolly took it to a tie-break, where he eased over the line to secure a deserved win.
"It's been a fantastic season, a very emotional one after what I've been going through with injuries". That's what I should have aimed for.
Nadal is due to play Belgium's David Goffin in his first match at the tournament on Monday, but told reporters that his injured knee might not hold up.