Defending champion Watson bows out in Japan
Britain's Heather Watson plays a shot against Varvara Lepchenko of the US at the AEGON International tennis tournament in Eastbourne, southern England on June 17, 2013
Puig, seeded eighth in the event and ranked 48th in the world, outlasted 92nd-placed Watson 6-4, 6-4 in 94 minutes.
Third-seeded Australian Samantha Stosur, the 2011 US Open champion, beat Johanna Larsson of Sweden 6-4, 6-2 to reach the second round of the tournament, carrying a total purse of $235,000.
The singles' field has been diluted by the 11th-hour withdrawal of top-seed Jelena Jankovic of Serbia due to pain in her lower back.
She was injured while losing 6-2, 6-2 to world number one Serena Williams in the final of the China Open in Beijing on Sunday, according to tournament organisers.
Last year's semi-finalist, Misaki Doi of Japan, whipped Spain's Silvia Soler-Espinosa 6-1, 6-2. Her compatriot Kurumi Nara defeated Caroline Garcia of France 6-3, 6-2.
Canadian Eugenie Bouchard, seeded fifth, trounced Varvara Lepchenko of the United States 6-3, 6-0 in the other first-round singles match on the tournament's opening day at the Utsubo Tennis Centre in Osaka.
The 21-year-old Watson beat Chang Kai-chen of Taiwan in full sets in her first-ever WTA final at the Japan Women's Open last year. She has not won a WTA event since.