Spain's Aitana Bonmati scored one goal and created another to earn the world champions a 2-1 comeback win over Japan, while the United States cruised to a 3-0 win over Zambia in the Olympic Games.
The Americans' victory gave former Chelsea boss Emma Hayes a winning start in her first major tournament with the team.
The US began their quest for a fifth gold medal with a resounding victory at Nice Stadium.
Trinity Rodman scored after a brilliant move in the 17th minute and Mallory Swanson found the net twice in a minute, between the 24th and 25th, to clinch their first three points in Group B.
Zambia were reduced to 10 players at the half-hour mark after a red card for Pauline Zulu, who left the pitch in tears.
Olympic debutants Spain recovered from going behind to an early goal in Nantes and beat Group C rivals Japan to avenge a 4-0 group stage loss to the Asians in last year's World Cup, which they went on to win.
Aoba Fujino's superb free kick opened the scoring for Japan, but Ballon d'Or winner Bonmati equalised only nine minutes later with an easy finish.
Spain dominated possession in the second half and sealed the win in the 74th minute courtesy of a Mariona Caldentey strike following a Bonmati pass to go top of the group.
"We have started the Games very well. I'm happy for the win. (The World Cup loss against) Japan left us with a thorn in our side," Bonmati said.
"The goal we conceded didn't change our mentality. We played our football and we deserved the win."
Elsewhere, reigning Olympic champions Canada also claimed a 2-1 comeback victory over New Zealand in St Etienne in the first day of action in Group A.
France's Marie-Antoinette Katoto struck twice to help the hosts to a 3-2 win over Colombia in their Group A opener in Lyon.
Katoto opened the scoring after six minutes, pouncing on a misplaced pass by Linda Caicedo.
Kenza Dali doubled the lead in the 18th minute before Katoto got her second and made it 3-0 with a header three minutes before halftime.
Colombia's Catalina Usme and Manuela Pavi then made it 3-2 scoring in the 54th and 64th minutes, respectively.
But their hopes of a comeback and a first Olympic win were dashed when Mayra Ramirez was shown a red card five minutes from time.
Germany made a strong start to the tournament in Marseille with a 3-0 win over Australia in Group B match, thanks to goals from Marina Hegering, Lea Schueller and Jule Brand.
Towering headers from Hegering in the 24th and Schueller in the 64th, as well as a Jule Brand effort sealed the rout for the 2016 Olympic champions.
Brazil earned a 1-0 win over Nigeria in Bordeaux in Group C. Gabi Nunes scored in the 37th minute following a pass from Brazil's all-time leading scorer Marta, who is playing at her sixth Olympics.