Jannik Sinner was in the locker room, getting ready to go on court and try to win a match that would give Italy its second Davis Cup title in a row, when his teammate, Matteo Berrettini, was asked about being so close to a championship. Berrettini, who won the first match of the best-of-three final against the Netherlands on Sunday, refused to take anything for granted. Not when Sinner, who is just 23, is the one swinging a racket.