Defending champions The Netherlands eased to a 3-0 win over Belgium and World Cup holders Argentina crushed South Africa 7-1 on the opening day of Olympic women's field hockey on Sunday.
New Zealand pulled off a 1-0 victory over former champions Australia, while hosts Great Britain hammered Japan 4-0.
Germany clinched a hard-fought 2-1 victory over the United States and China outplayed South Korea 4-0.
Kim Lammers fired two field goals and Caia van Maasakker capitalised on a late penalty corner to launch the Dutch campaign with a sound win over hard-working Belgium in Pool A.
The Dutch dominated the game against Belgium, but were kept at bay until Lammers opened the scoring in the 33rd minute when she shot in after picking up a long pass from Eva de Goode.
Lammers was again on the mark in the 42nd minute off a pass from Ellen Hogg before van Maasakker rounded off the scoring in the 60th with a low penalty corner drive.
Captain Luciana Aymar converted two penalty corners in the first session to lead Argentina's scoring spree against the South Africans.
Fourth-ranked Great Britain scored all their four goals in the initial 28 minutes of their Pool A match with Japan.
Star striker Alex Danson fired the first and fourth goals, with Sarah Thomas and Sally Walton scoring in between.
Britain seemed content to maintain sufficient control and not allow Japan a single chance in the game.
Their victory may have come at a cost as captain Kate Walsh was accidentally hit in the face by a Japanese stick four minutes from the end.
She was taken to hospital after the match.
Josefina Sruoga, Silvina D'Elia and Noel Barrionuevo scored from penalty corner action while Martina Cavallero and Carla Rebecchi fired in field goals as Argentina stamped their authority over South Africa in the Pool B contest.
The South Africans got a consolation goal through Dirkie Chamberlain on a counter-attack.
New Zealand rode on an early goal from Cathryn Finlayson in the Pool B match to score their maiden victory in Olympic Games against their Oceanic rivals.
It was the first time the Black Sticks had taken even a point off Australia in the Olympics. They had always lost by at least three goals in the past.
Finlayson scored from a corner in the third minute with a follow-up shot from eight yards after the initial drive by New Zealand captain Kayla Sharland had been saved.
New Zealand were the more cohesive team but both sides created enough chances to have added to the score.
New Zealand coach Mark Hager said: "It was not pretty, we were lucky. They hit the post."
Australian coach Adam Commens was disappointed with his team's wastefulness.
"We created enough chances, and when you do so you should be able put the ball in," he said.
Germany staved off a late United States resurgence to prevail 2-1 in their Pool B encounter.
German captain Fanny Rinne opened the scoring in the eighth minute with a rising penalty corner flick into the net and Lisa Hahn sounded the boards with a reverse drive after receiving Janne Mueller-Wieland's pass inside the circle in the 21st.
Trailing by two goals at half-time, USA came back strongly, but only managed to score one goal when Lauren Crandall scored during a 56th minute penalty corner.
China came back to form after two lean years with what their coach Kim Sang Ryul described as an "unbelievable" win over South Korea in Pool A.
China had not beaten their Asian rivals in their previous nine matches.
Kim said "Everything went right for us today. I have had faith in this team. Others doubted me. Such a result against Korea."
Ma Yibo helped by scoring her side's first and last goals with penalty corner shots, the others came from Zhao Yudiao and Li Xongxia.