ON DUPLICATE KEY UPDATE with WHERE condition

I update/insert values in a single table with the ON DUPLICATE KEY UPDATE function. So far everything is fine.

INSERT INTO table1 SET field1=aa, field2=bb, field3=cc
ON DUPLICATE KEY UPDATE SET field1=aa, field2=bb, field3=cc;

But now I would like to achieve that the update only is done if a condition (WHERE) is true.

Syntactically not correct:

INSERT INTO table1 SET field1=aa, field2=bb, field3=cc
ON DUPLICATE KEY UPDATE SET field1=aa, field2=bb, field3=cc WHERE field4=zz;

Any ideas how the correct SQL statement is?

Thanks a lot.