Stefanos Tsitsipas beat Daniil Medvedev 7-6(5) 6-4 in a high-quality battle of the debutants at the ATP Finals on Monday.
Tsitsipas, the first Greek player to qualify for the tournament, had lost all five previous matches against the Russian, but turned the tables in impressive fashion.
The 21-year-old served superbly and did not face a single break point and took his chances in clinical fashion to the delight of the O2 Arena crowd who clearly enjoyed his style.
Playing the more adventurous tennis, Tsitsipas earned a set point in the opening set tiebreak with a volley onto the sideline and converted when Medvedev netted a forehand.
Striking the ball with venom, the swashbuckling Tsitsipas earned three break points at 3-3 in the second set but Medvedev responded with some gutsy tennis to hang on.
A misjudgement by Medvedev at 4-4, leaving a ball that landed on the baseline, gave Tsitsipas another break point and this time he took his chance.
Tsitsipas, who won 39 of 44 points when he landed his first serve, claimed victory with a smash.
"I had goosebumps today, playimg on this court that I dreamed of playing on, it's pure joy," Tsitsipas said.
"This was one of my most important victories of my career so far. Everything worked pretty well.
"Not easy coming in knowing you have lost five times but I made a deal with myself that I would keep trying."
Later in Monday world number one Rafael Nadal takes on last year's champion Alexander Zverev. [Reuters]