Nishikori, the number 14 seed, who became the first Japanese player to win the Japan Open on Sunday, eased past China's Wu Di 6-2, 6-4 in his first round match.
"Obviously, it's not easy to play (so soon) after winning the tournament," said the 22-year-old, who is at career-high of number 15 in the rankings after his success in front of home fans.
"But I had a good day today. I love to play here," added Nishikori, who reached the semi-finals in Shanghai last year before he lost to eventual champion Andy Murray of Britain.
Nishikori's weekend win over big-serving Milos Raonic was his first on the Tour since the 2008 Delray Beach title, sharply raising expectations that Asia had uncovered a potential world-beater.
Eighth seed John Isner was forced to dig deep to save three match points to win a second-round match containing three tie-breaks and keep alive his outside chance to qualify for next month's ATP World Tour Finals.
Isner booked his place in the third round by beating South Africa's Kevin Anderson 7-6 (7/3), 6-7 (8/10), 7-6 (9/7) in two hours and 45 minutes.
In a match featuring 45 aces and no service breaks, Anderson squandered match points at 6-4 and 7-6 in the third set tie-break.
"Really we both served well," said Isner. "It's one of the few times where I don't think I was out served, but he served right up there with me as far as, you know, aces, percentages, whatnot."
"It came down to a few points here or there, and I was able to pull it out. It was looking bleak for a little bit," he added.
In first-round action, French 15th seed Gilles Simon knocked out Jurgen Melzer of Austria 6-4, 6-2 and Switzerland's Stanislas Wawrinka, the number 13 seed, battled past Albert Ramos of Spain 7-6 (7/1), 7-6 (7/5).
The top three seeds -- Roger Federer, Novak Djokovic and Murray -- all open their campaigns on Wednesday after first-round byes for the top eight seeds.
Shanghai Masters: Kei Nishikori wins, John Isner digs deep
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