Roger Federer and young gun Alexander Zverev lit up London's O2 Arena with a ridiculous rally that the Swiss eventually claimed on his way to another ATP World Tour Finals win.
Federer backed up his opening victory over American Jack Sock with another classy display against the 20-year-old German to become the first man through to the semi-finals of the event.
The world number two moved into his final group match with a 7-6 5-7 6-1 victory in just under two-hours and 13 minutes.
A thrilling opening set was highlighted by one of the points of the match at when Federer won a lengthy rally while 6-5 up.
Both players were locked at deuce when Federer sent Zverev scurrying to the net off a beautifully crafted drop shot.
The German's managed to return the shot cross-court before Federer sent him scrambling back to the baseline on the opposite side of the court.
Again, Zverev returned the Swiss star's shot and the youngster thought he had the crucial point won when Federer hit a long cross-court forehand that appeared to go long.
The German immediately called for a challenge after the umpire called it in but unfortunately for the German, Hawk-eye confirmed the original call and Federer went on to win the set.
To his credit, Zverev showed tremendous resilience to bounce back from the opening set heartbreak as he edged Federer in a gripping second set 7-5.
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.
The Swiss re-found his focus and some consistency on serve in the decider as Zverev's level dipped, earning three breaks to close it out 6-1.
Zverev has been touted as the leader of a pack of young players hungry to oust the creaking old guard but Federer, who has suffered only four defeats all year, shows no sign of slipping.
"It was a good battle and I always enjoy myself at this beautiful arena," said Federer, who has now reached the semi-finals 14 times.
"I have so much fun playing in London and I have been so successful here. Thank you for making it such a special night.
"I'm happy and relieved that I can play freely against (Marin) Cilic, instead of it being a nailbiter. It's been a tough group so to be through in two matches is great."
Federer now faces Marin Cilic in his final round-robin match, while Zverev will take on American Jack Sock.
World number nine Sock mounted a spirited fightback to claim a first ATP World Tour Finals victory that leaves Cilic facing an early exit.
Sock defied the odds to qualify for the season-ending tournament for the first time and boosted his chances of reaching the semi-finals with a 5-7 6-2 7-6 (7-4) victory over the fifth seed in Group Boris Becker.
Both players could ill afford another defeat after losing their first matches at the O2 Arena and there was a sense of deja vu for Cilic, who led Zverev 3-1 in the final set of his opening contest before being consigned to defeat.
The Croatian led Sock by the same margin in the decisive set on Tuesday and was 4-2 up in the tie-break, but the eighth seed from America refused to be beaten and has now won all three matches against Cilic.