London - Stefanos Tsitsipas revelled in his victory against Daniil Medvedev at the ATP Finals on Monday, admitting their spiky relationship had made his job tougher as Rafael Nadal prepared to make his bow.
The Greek 21-year-old came into the match at London's O2 Arena with a 5-0 losing record against his Russian opponent weighing on him.
But 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 (7/5), 6-4 victory.
The two players have had a rocky relationship.
Tensions flared between them in Miami last year and Tsitsipas recently labelled Medvedev's way of winning as "boring" after defeat against the Russian in Shanghai.
Tsitsipas said Medvedev had got "into my head" in Miami - he said the row had centred on a demand from the Russian for an apology over a net cord.
"I mean, our chemistry definitely isn't the best that you can find on the Tour," said Tsitsipas, who added that Monday's match had been tougher because he was so desperate to win.
"It just happens with people, you can't just like everyone. It's not that I hate him. I guess - yeah, as he said, we will not go to dinner together, so... I respect him, for sure."
The Greek, supported vociferously at the O2 Arena, could not convert the lone break point against the US Open finalist in the opening set and it went to a tie-break.
But at 5-5 in the breaker, Tsitsipas produced one of best points of the match and finished it with a forehand volley winner. He converted the opportunity with an aggressive forehand and let out a roar of delight.
The second set remained tight but Medvedev fatally opted to let a forehand go when rushing the net at 4-4 and it landed well inside the baseline, giving the Greek a break point.
Medvedev then hit a backhand long to give Tsitsipas the first break of the match and he did not let his chance slip, serving out for victory.- Next Gen challengers -
World number four Medvedev, 23, has emerged as the leader of the pack of young tyros bidding to unseat the old guard of Nadal, Novak Djokovic and Roger Federer.
Four of the eight competitors at the ATP Finals at the O2 Arena are under 24 for the first time in 10 years.
Nadal, takes on defending champion Alexander Zverev in the later match in Group Andre Agassi with major questions over his fitness since he withdrew injured from the Paris Masters at the semi-final stage.
The Spanish top seed is locked in a battle with Djokovic to finish as the year-end number one but was not serving flat out in practice last week due to an abdominal strain.
Regardless of Djokovic's results this week, Nadal will clinch the year-end top spot for a fifth time if he reaches the final with a 4-0 record.
Djokovic launched his bid for a sixth ATP Finals title with a comfortable win against Matteo Berrettini on Sunday but Federer slipped to a straight-sets defeat against Dominic Thiem, putting his hopes of progress in serious jeopardy.
Results on the second day of the ATP Finals at London's O2 Arena on Monday (x denotes seeding):
Group Andre Agassi
Stefanos Tsitsipas (GRE x6) bt Daniil Medvedev (RUS x4) 7-6 (7/5), 6-4
Rafael Nadal (ESP x1) v Alexander Zverev (GER x7)
Group Max Mirnyi
Kevin Krawietz/Andreas Mies (GER/GER x3) bt Jean-Julien Rojer/Horia Tecau (NED/ROU x6) 7-6 (7/3), 4-6, 10-6
Juan Sebastian Cabal/Robert Farah (COL/COL x1) v Pierre-Hugues Herbert/Nicolas Mahut (FRA/FRA x7)