Belgium took a surprise early lead when Jerome Dekeyser converted a penalty corner in the 4th minute and Florian Fuchs equalized from open play nine minutes later before Christopher Zeller got the decisive goal in the 45th.
Germany weathered a torrid closing 15 minutes as Belgium pushed for an equalizer.
The Netherlands beat eight-time Olympic champion India 3-2 in the day's opening match and Australia skipper Jamie Dwyer scored a hat-trick to help his No. 1-ranked side defeat South Africa 6-0.
The Netherlands pulled ahead of India early after Roderick Weusthof scored from a penalty corner. Robert van der Horst scored next to help his side end the first half 2-0 ahead.
India, which dominated men's hockey for almost half a century but has been in a slump for years, started the second half strongly with forwards Dharamvir Singh and Sarvanjit Singh finding the net in the 45th and 48th minute to level the scores. Mink van de Weerden scored a winner for the Netherlands.
"Our forward line was poor and we must improve on our finishing," said India midfielder Sardar Singh. "We made a good comeback in the second half. We took our chances and we felt more in control of the match."
Australia encountered little resistance from the South Africans.
"It opened up a little bit in the second half and we capitalized with a few goals," Dwyer said. "Overall it was a pretty good result, but we weren't thorough enough in a lot of areas and we need to improve."
"I had two (penalty) strokes in there so you are meant to score those goals. I'm glad I executed the two strokes properly and the penalty corner rebound, so overall very happy to get my name on the score sheet," Dwyer said.
In another match, Pakistan held the Beijing silver medallist Spain to a 1-1 draw.
Spain captain Santiago Freixa broke his arm in a collision with Muhammad Irfan late in the first half and though he tried to play on in the second half he was clearly in pain. He has been ruled out for the rest of the tournament.
Both sides created many chances, but only forward Rehan Butt converted for Pakistan in the 46th minute and Pau Quemada scored for Spain a minute later.
Earlier, You Hyo-sik scored both goals as South Korea beat New Zealand 2-0.
Captain Barry Middleton scored twice as host nation Britain overpowered Argentina 4-1.
Dan Fox and Richard Smith also scored as Britain raced to a 4-0 lead before Pedro Ibarra converted a penalty corner for Argentina.
"We had a lot of shots on target and quite a few balls flashing across goal, so if you put those away in international hockey you win by three or four goals every game," Fox said.