Reigning ATP Finals champion Alexander Zverev tasted his first victory over world No 1 Rafael Nadal with a scintillating 6-2, 6-4 triumph in London.
The Spaniard, who has yet to win this season-ending event, showed no sign of the abdominal injury sustained in Paris this month but was comprehensively beaten by Zverev in his opening match.
"It was not a problem with the abdominal at all," said the 33-year-old Nadal, who will next face Daniil Medvedev in his second round-robin match in a repeat of the US Open final.
"I did not feel pain. We can find reasons or excuses, but at the end of the day, all that really matters is I need to play much better in two days.
"I needed my best competitive spirit today, and I was not there in that way."
Zverev broke serve in the fifth game and again two games later when Nadal blazed a wild forehand wide.
Nadal dropped his serve again at the start of the second set and with Zverev rock solid on his own delivery he never allowed the Spaniard a glimpse of an opportunity.
The statistics told the story of the match with Zverev striking 26 winners to Nadal's 13 and winning 30 of 34 points behind his first serve.
Zverev, 22, has salvaged what initially looked like being a poor year with a late-season flourish to qualify for the showpiece.
The world No 7 has now beaten Roger Federer, Novak Djokovic and Nadal in consecutive matches at the O2 Arena where last year he claimed the biggest title of his career.
"It is obviously great," Zverev said. "Everyone knows how much I have struggled the whole season. It means so much to be playing here again where I won my biggest title last season. It means everything to me."
In terms of Nadal's hopes of arriving in Abu Dhabi for the Mubadala World Tennis Championship next year as world No 1 for a fifth time, nothing has changed.
He arrived in London with a 640-point lead over Djokovic. With 200 points for each group win, Djokovic cut that deficit on Sunday by thrashing Italian Matteo Berrettini.
Even if Nadal suffered a recurrence of the injury or lost all of his group matches, Djokovic must still reach the final to overtake the Spaniard and match Pete Sampras's record of finishing a year as the top-ranked player on six occasions.
Djokovic, seeking a record-equalling sixth ATP Finals title, can narrow the gap further on Tuesday when he takes on Austrian Dominic Thiem.
Zverev faces Stefanos Tsitsipas next, who earlier on Sunday got the better of Russian bogeyman Daniil Medvedev.
The Greek 21-year-old had lost all five of his previous encounters against Russia's Medvedev.
He edged a tight first set, winning the tie-break, and a single break late in the second set proved decisive in a 7-6, 6-4 victory.
Federer is in action on Tuesday when the 20-time Grand Slam champion faces Berrettini.