Sinner capped off a successful season, winning her 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 over the past 12 months.
In a press conference after winning the Davis Cup, Berrettini described Sinner as “the most humble man on the planet.”
However, Sinner’s year has been disrupted by the ongoing doping controversy, which has left his immediate future in the sport uncertain.
“Of course it’s in my head a little bit,” Sinner said of the doping scandal.
“The best part is that everyone who knows me as a person trusts me, so I kept playing at the level I was at.
“I was a little bit emotionally discouraged and a little heartbroken. Sometimes life throws challenges at you, it’s bound to happen. [deal with] that.”
Sinner looked tired as he was forced into a tie-break in the opening set against Griekspoor, but surged late in the second game to seal Italy’s third Davis Cup title on his fourth match point.
Italy become the sixth team to win back-to-back Davis Cup titles since the final stage was introduced 52 years ago, and the first since the Czech Republic in 2013.