Nakashima, ranked 63rd in the world, broke serve four times and won when the 91st-ranked Cerúndolo returned a serve into the net.
All four players are in Group A of the eight-man tournament, which has a round-robin format. The top two in both groups qualify for Friday’s semifinals. The final is on Saturday.
In Group B, Sebastian Korda of the U.S. plays Hugo Gaston of France in the first match of the evening session, followed by home favorite Lorenzo Musetti against Sebastian Baez of Argentina.
It is the fourth edition of the tournament in Milan for the top 21-and-under players on the ATP Tour. The tournament was canceled last year because of the coronavirus pandemic. The ATP Finals will also be in Italy, in Turin next week.
There are different rules at the Next Gen Finals, including on-court coaching, no-Ad scoring, medical timeout limits, and Hawk-Eye making all the line calls.
The most drastic change is the shorter set, where the first to four games takes the set, with a tiebreak at 3-3.
This year there are also shorter warm-ups, ensuring matches begin just one minute after the players enter the court, while bathroom breaks are timed to three minutes.