Australia, who lost all three of their World Cup group phase matches, had a chance to equalise five minutes later, but Belgium keeper Thibaut Courtois dived to his right to keep out a Tim Cahill shot.
Witsel made it 2-0 in the 77th minute, cracking a firm left-foot shot from the edge of the penalty area just beyond the outstretched Ryan.
"The first half was almost perfect... Everyone knew exactly what they had to do," said Belgium coach Marc Wilmots, before adding that he was particularly encouraged by his players' tactical flexibility.
Yet captain Vincent Kompany, who required a calf scan earlier in the week, lasted the full 90 minutes.
Belgium were due to kick off their Euro 2016 qualifying campaign against Israel on Sept. 9, but the match was postponed until March 31 because of security concerns.
Their first Group B fixture will now be against Andorra on Oct. 10. Bosnia, Cyprus and Wales are also in the Group.
Australia face Saudi Arabia at Fulham's Craven Cottage in London next Monday and host the Asian Cup in January.
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