Federer and Zverev had already met in the O2 Arena - when the German's practice session overran and his opponent was forced to bounce on his toes for a few minutes while Juan Carlos Ferrero put his man through the last couple of serves. "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".
The German then faced questions of his own as he faced a break point after dumping a forehand into the net - which left him standing with his hands on his hips - but some big serving kept him level in the first.
"I'n happy and relieved that I can play freely against (Marin) Cilic, instead of it being a nailbiter".
The 20-year-old third seed raced into a 4-0 advantage only to see the Swiss charge back to lead 5-4. The opening match in the Pete Sampras Group produced a feast of stylish backhands from both players but it was Dimitrov, the first Bulgarian to qualify for the tournament in its 48-year-old history, who overcome some late jitters to prevail.
Federer looked in control when he broke in the opening game of the second set, having claimed the first via a tiebreak.
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.
Jack Sock was quizzed thoroughly at the ATP Finals in London on all things British. "It's been an interesting morning, a fire alarm went off at 4am and we all had to exit our hotel so it was a long morning". As the world's most formidable player on clay, Nadal has in the past suggested it would be better if the season-ending Tour Finals at least occasionally took place on his preferred surface. It has mostly been played on an indoor hard-court, with other stints on carpet, grass and outdoor hard-courts when it was staged in various cities before setting up home in London in 2009.
"It was a good battle but I always enjoy myself coming out to the O2", said Federer in his courtside interview.