Mike Havenaar, Shinji Okazaki and midfielder Shinji Kagawa scored two apiece in Osaka as hapless Tajikistan suffered a backlash from Japan's recent dip in form.
"We knew we were expected to win but it felt good to win in such emphatic fashion," Japan coach Alberto Zaccheroni said in a pitchside interview.
"We expect goals from Shinji [Kagawa] and Okazaki but Havenaar deserves a mention too. All the players stepped up tonight and performed very well."
Beanpole striker Havenaar opened the scoring after 11 minutes with a bullet header, his first goal for Japan throwing the floodgates open.
Okazaki and Yuichi Komano then found the net before an audacious flick from midfielder Kagawa made it 4-0 just before half-time.
Japan had struggled in their first two Asian Group C qualifiers, scraping past North Korea 1-0 at home after an injury-time goal and escaping with a lucky 1-1 draw in Uzbekistan.
Uzbekistan beat North Korea 1-0 in Pyongyang earlier on Tuesday to keep pace with Japan on seven points.
However, the Blue Samurai put plenty of daylight between the teams in terms of goal difference, the Peter Crouch-like Havenaar scoring his second before Nakamura Kengo made it 6-0.
Kagawa added a fine curling effort in the 67th minute before Okazaki poached his second, his ninth goal in six games under Italian Zaccheroni, still unbeaten in 15 games since taking charge last October.
"That was deliberate," smiled Borussia Dortmund's Kagawa, who had been suffering from a recent crisis of confidence, when asked about his second goal.
"It was a shot. To score at what used to be my home stadium was special," added Kagawa, who previously played at Nagai Stadium for Cerezo Osaka. "It will give me a real lift."
