How to use outer join in update

In my current DBMS I can use

create table t1 ( f1 int, f2 int );
create table t2 ( f3 int, f4 int );
update t1 set f1=t2.f3 from t1 left join t2 on t1.f2=t2.f4

This does not work in Postgres.

How to convert this statement to Postgres 8.1 ?

Andrus.