Federer up and running
Roger Federer kickstarted his bid to win a seventh ATP World Tour Finals title - and his first since 2011 - with a 6-4, 7-6(4) victory over event debutant Jack Sock.
Having qualified courtesy of his maiden Masters 1000 title in Paris, Sock was initially - and understandably - nervous here in London, and Federer capitalised with a break of serve in the very first game.
The American did regain his poise, and had some success with his powerful forehand, but was unable to restore parity as the opener went the way of the Swiss.
The second set was a tighter affair, but although Sock was growing in confidence, he was unable to make the returns that would trouble the Federer serve; in all, the 36-year-old just dropped 11 points in 11 service games.
In truth, the reigning Wimbledon singles champion had set a course for victory early on in the encounter, and never looked likely to be derailed.
By way of contrast, Sock was made to work hard for his service holds, but stuck to his task admirably, frustrating Federer as he fended off a pair of break points at 3-3, two more at 4-4 and then another at 5-5.
The pair arrived at a tie-break, and though Sock did recover from a mini-break to get to 4-4, he promptly handed the initiative back to his opponent with a double-fault, and Federer served out the encounter for his 50th match win of the season.
"I'm happy that I was able to come out today and had actually good energy," said Federer, who is the oldest man to qualify for the ATP's season-ending finale since 1970.
"This is the best I've felt since the Del Potro final (in Basel). I'm very happy to see that I didn't have to pay the price for taking it easy and resting and recovering.
"I think we're all going to start playing better every round that goes by."
Zverev battles back for victory
Alexander Zverev marked his debut at the ATP World Tour Finals with a hard-fought, 6-4, 3-6, 6-4 triumph over Wimbledon singles finalist Marin Cilic.
The 20-year-old admitted he had been "a little bit nervous" as he entered the fray, but it was far from evident in the early exchanges. A player seemingly tailor-made for the big stages, Zverev broke Cilic at the first opportunity thanks to an exquisite drop volley, and kept his opponent at bay throughout the opening set with an aggressive baseline display.
Having hit 12 winners in the opening set, Zverev's level began to drop off in the second, and Cilic capitalised. Upping the velocity and variety of his strokes, the Croatian raced out to a 3-0 lead from which Zverev would be unable to recover.
The momentum was undoubtedly with Cilic, and when he broke and consolidated for a 3-1 lead in the decider, the match appeared to be in his grasp.
But much about Zverev's game belies his tender years, and it was at that point that he began displaying the poise and confidence in his own game that has seen him touted as a future world No.1.
Allying power with accuracy, the world No.3 staged an impressive comeback, breaking Cilic twice as he rattled off five of the remaining six games to claim a maiden victory at the ATP World Tour Finals.
"For the most part of the second and third set he was the better player," Zverev said after the match.
"I was just happy to come back and win. If you have the opportunity to play here you have to take it."
Next up for Zverev is Federer, with the winner of that encounter set to assume control of the Boris Becker group. The pair have met twice this year, with their head-to-head tied at a set apiece.
Even Federer misses the target sometimes...
"It was a big distraction because it was very big," said Federer of Sock's diversionary tactics.
"That's what I should have aimed for. That target was bigger than the down-the-line court that I had!"