TestNG : run tests sequentially

I have a problem to configure in a correct way my testng.xml.

I want to run all my test sequentially. One by one.

in my xml file I have this code :

<test name="Init" preserve-order="true" parallel="false">
    <classes>
        <class name="tests.Init" />
        <class name="tests.test1" />
        <class name="tests.test2" />
    </classes>
</test>

but when I run testng, all the three tests run at the same time.

How can I set testng to launch one test after another ?