If you cannot use spark-csv, you can do the following:
df.rdd.map(lambda x: ",".join(map(str, x))).coalesce(1).saveAsTextFile("file.csv")
If you need to handle strings with linebreaks or comma that will not work. Use this:
import csv
import cStringIO
def row2csv(row):
buffer = cStringIO.StringIO()
writer = csv.writer(buffer)
writer.writerow([str(s).encode("utf-8") for s in row])
buffer.seek(0)
return buffer.read().strip()
df.rdd.map(row2csv).coalesce(1).saveAsTextFile("file.csv")