What is currently the longest commercial flight in terms of distance? Flightradar24 has announced few hours ago, that Qantas QF7 is taking off for its 13,804 km long flight and named it the longest commercial flight (by distance).

Is that correct? I was more than sure, that I heard few years ago, that current record belongs to Singapore Airlines, that has at least one commercial route of a little bit more than 14k kilometers.
According to Wikipedia, the flight you are referring to, SQ21, got discontinued in November 2013. The runner up, still from Singapore Airlines, SQ37, got discontinued one month earlier.

The same page reports that QF8 is the current holder of the title of "longest non-stop flight" and has the same destinations as today's QF7 as of February 2017 Qatar Airways flights 920/921, covering 14,534 km (7,848nm / 9,031mi) from Doha, Qatar (DOH/OTBD) to Auckland, New Zealand (AKL/NZAA), beats QF8 by about 395 NM.

If we instead include 1-time-only flight by a commercial airliner, the longest one has been flown in November 2005 between Hong Kong and London, as a demo by Boeing.

The title of absolute longest flight without refueling is instead held by a non-commercial flight done in an aircraft specifically built for the purpose.