The second-ranked Spaniard beat Fish 6-2, 3-6, 7-6 (7-3) in his first group match at the season-ending event at the O2 arena in London.
Nadal, a runner-up to Roger Federer last year, is seeking his first ATP Finals title. Fish, the lone American in the field, made his debut at the event for the top eight top singles and doubles players.
Earlier today, defending champion Federer opened the London Finals at the O2 arena with a 6-2, 2-6, 6-4 win against France’s Jo-Wilfried Tsonga.
The ATP Finals are played in a round-robin format, with the top eight singles and doubles players on the ATP rankings divided into two groups. Each will face the three others in the group, with the top two in each section moving into the knockout semifinals.
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