For Sinner, it capped a successful season that saw him capture his first two Grand Slam titles at the Australian Open in January and the US Open in September.
The 23-year-old, who also won the season-ending ATP Finals earlier this month, has lost just six of his 76 ATP Tour matches in the last 12 months.
Speaking in a press conference after their Davis Cup victory, Berrettini described Sinner as “the most humble person on the planet”.
However, Sinner’s year has been disrupted by an ongoing doping controversy that has left him behind his future in the sport is uncertain.
“Of course it was a little bit in the head,” Sinner said of the doping case.
“The most important part is that everyone who knows me as a person believes in me – that’s why I continue to play at the level that I have.
“I’m emotionally a little sad, a little heartbroken. Sometimes life gives you adversity and you just have to (deal with it).”
Sinner looked exhausted when he was forced into a tie-break in the opening set against Griekspoor, but recovered late in the second to seal Italy’s third Davis Cup victory on his fourth match point.
Italy became the sixth team to win back-to-back Davis Cup titles since the final was introduced 52 years ago, and the first since the Czech Republic in 2013.
