  The connection between real-time quantum field theory (RTQFT) [see, e.g., A.\
Kamenev and A.\ Levchenko, Advances in Physics {58} (2009) 197] and phase-space
techniques [E.\ Wolf and L.\ Mandel, {\em Optical Coherence and Quantum Optics}
(Cambridge, 1995)] is investigated. The Keldysh rotation that forms the basis
of RTQFT is shown to be a phase-space mapping of the quantum system based on
the symmetric (Weyl) ordering. Following this observation, we define
generalised Keldysh rotations based on the class of operator orderings
introduced by Cahill and Glauber [Phys.\ Rev.\ {177} (1969) 1882]. Each
rotation is a phase-space mapping, generalising the corresponding ordering from
free to interacting fields. In particular, response transformation [L.P.\ and
S.S., Ann.\ Phys. (N.Y.) {323} (2008) 1989] extends the normal ordering of
free-field operators to the time-normal ordering of Heisenberg\ operators.
Structural properties of the response transformation, such as its association
with the nonlinear quantum response problem and the related causality
properties, hold for all generalised Keldysh rotations.
  Furthermore, we argue that response transformation is especially suited for
RTQFT formulation of spatial, in particular, relativistic, problems, because it
extends cancellation of zero-point fluctuations, characteristic of the normal
ordering, to interacting fields. As an example, we consider quantised
electromagnetic\ field in the Dirac sea. In the time-normally-ordered
representation, dynamics of the field looks essentially classical (fields
radiated by currents), without any contribution from zero-point fluctuations.
For comparison, we calculate zero-point fluctuations of the interacting
electromagnetic\ field under orderings other than time-normal. The resulting
expression is physically inconsistent: it does not obey the Lorentz condition,
nor Maxwell's equations.
