After two games each, Japan and China PR are both on four points from one win and one draw with identical goal difference. Korea Republic have three points while Hong Kong have zero after two straight defeats.
"I was able to stay calm and score the first goal as they made a mistake and the ball came to me. I was also calm to score my second goal. I needed a good result, so I'm satisfied with it," said Tamada.
The first goal came in the 41st minute when the Nagoya Grampus striker quickly collected a poor clearance by Hong Kong defender Chan Wai-ho and fired home from near the left corner.
Brazil-born defender Marcus Tulio Tanaka doubled the advantage when he leaped highest for a header from midfielder Yasuhito Endo's corner in the 59th minute.
Another Endo corner kick sent the ball into the area dangerously in the 82nd minute with skipper Yuji Nakazawa and substitute forward Sota Hirayama hitting shots before Tamada netted his second in a goal-mouth confusion.
Tulio hit the first shot on goal from an Endo corner in the 15th minute, and he was a consistent thorn in the side of the defence as the Japanese kept goalkeeper Yapp Hung-Fai busy.
Hong Kong's best chance came at the beginning of the second half when midfielder Xu Deshuai's dangerous-looking free kick was caught by Japanese goalkeeper Seigo Narazaki.
"We wanted to score a couple of more goals, so I'm a bit disappointed. We tried to play our own game with rhythm," said Japan coach Takeshi Okada.
Meanwhile, Hong Kong's Korean coach Kim Pan-Gon apologised to supporters because his team once again failed to score in the tournament. They lost their first match to Korea Republic 5-0, but Kim was looking back to a pair of lopsided defeats (6-0 and 4-0) to Japan at the end of last year.
"We reduced the number of goals conceded, but we are still too far behind Japan," said Kim. "We were only defending. We lost the ball too quickly."