Stefanos Tsitsipas built on the momentum from his maiden ATP title at the weekend to reach the second round of the Swiss Indoors on Wednesday with a 6-2, 7-6 (7/3) win over Jeremy Chardy.
Tsitsipas, the first Greek to lift an ATP trophy when he beat Ernests Gulbis in Stockholm on Sunday, earned his 40th victory of the season by seeing off longtime training partner Chardy.
Tsitsipas took revenge over Chardy for a loss to the Frenchman in Montpellier in February.
The 20-year-old had accumulated only four career tour-level match victories coming into 2018, but has enjoyed a breakthrough season and will take on Germany's Peter Gojowczyk next in Basel.
Second seed Alexander Zverev notched his tour-leading 50th win of the season as he brushed past Dutchman Robin Haase 6-4, 7-5 in his debut at the event.
The German, who has secured a spot in the ATP Finals in London next month, managed to overcome 19 unforced errors in the opening set.
Zverev will face Australian qualifier Alexei Popyrin in round two.
However, 2016 champion and third seed Marin Cilic crashed out 7-5, 7-6 (7/2) to Romania's Marius Copil in his second-round match.
Taylor Fritz beat Swiss wild card Henri Laaksonen 6-2, 7-5 and will meet Copil in the last eight.
Latvia's Gulbis received on-court treatment around his hip muscle area on the way to a 7-5, 6-4 victory over fifth seed Jack Sock.
The defeat dropped the American's record this year to a dreadful 7-20 as he lost for the sixth time in his last seven matches.
Spanish eighth seed Roberto Bautista Agut defeated Portugal's Joao Sousa 6-4, 6-3, while Serbian lucky loser Dusan Lajovic beat Adrian Mannarino of France 6-7 (4/7), 6-1, 6-2.