Tomiyasu nodded in a 20th minute corner in a rare attack for Japan as the three-time champion Saudis dominated the first half without really threatening Shuichi Gonda's goal.
After the break the pattern largely continued and the toothless Saudis made a meek exit from the United Arab Emirates. Japan will continue their bid for a fifth continental crown against surprise package Vietnam in Dubai on Thursday.
Later Monday the UAE face Kyrgyzstan in Abu Dhabi with the winner meeting whoever triumphs between reigning champions Australia and Uzbekistan in Al Ain.
Japan coach Hajime Moriyasu made 10 changes to the rotated side which defeated Uzbekistan to top Group F game with only Yoshinori Muto keeping his spot in Sharjah.
The reassembled team took time to settle but snatched the lead with their first chance when Tomiyasu headed in Gaku Shibasaki's corner.
Saudi Arabia controlled 70 per cent of possession in the first half but Gonda did not have a save to make, though he was forced to scramble as Hatan Bahbri's curler dropped just the wrong side of the post.
Muto spurned a breakaway but even when Saudi Arabia started creating chances Salem Al Dawsari, from the middle of the box, and Ali Al-Bulayhi, with a free header, couldn't hit the target.
Japan were pushed deep in injury time but held out relatively comfortably.
After the break the pattern largely continued and the toothless Saudis made a meek exit from the United Arab Emirates. Japan will continue their bid for a fifth continental crown against surprise package Vietnam in Dubai on Thursday.
Later Monday the UAE face Kyrgyzstan in Abu Dhabi with the winner meeting whoever triumphs between reigning champions Australia and Uzbekistan in Al Ain.
Japan coach Hajime Moriyasu made 10 changes to the rotated side which defeated Uzbekistan to top Group F game with only Yoshinori Muto keeping his spot in Sharjah.
The reassembled team took time to settle but snatched the lead with their first chance when Tomiyasu headed in Gaku Shibasaki's corner.
Saudi Arabia controlled 70 per cent of possession in the first half but Gonda did not have a save to make, though he was forced to scramble as Hatan Bahbri's curler dropped just the wrong side of the post.
Muto spurned a breakaway but even when Saudi Arabia started creating chances Salem Al Dawsari, from the middle of the box, and Ali Al-Bulayhi, with a free header, couldn't hit the target.
Japan were pushed deep in injury time but held out relatively comfortably.