Is there always only one record in f1? If not, can there be duplicates in f1? Show a better example with more records in f1, including duplicates, if that's possible.
Are the records in f1 already in order by the key (1-5)?
Are the records in f2 already in order by the key (6-10)?
What is the RECFM and LRECL of each input file?
Run this job and show the //SYSOUT messages you receive, so I can see what level you're at:
//S1 EXEC PGM=SORT
//SYSOUT DD SYSOUT=*
//SORTIN DD *
//SORTOUT DD DUMMY
//SYSIN DD *