Reduce space in TOC between section groups

I have the following TOC:

section 1
subsection 1.1 ...................... 2

section 2 subsection 2.1 ...................... 2

How can i reduce the spacing between the first 'group' of items related to section 1 and between the second 'group' of items related to section 2?

I know how to decrease the spacing between items of the same group, e.g., between all subsections of section 2.

I've seen similar posts but i couldn't apply the provided solutions.

This is defined as part of the function that sets the \section heading in the ToC, known as \l@section. Here's its definition inside article.cls:

\newcommand*\l@section[2]{%
\ifnum \c@tocdepth >\z@
\setlength\@tempdima{1.5em}%
\begingroup
\parindent \z@ \rightskip \@pnumwidth
\parfillskip -\@pnumwidth
\leavevmode \bfseries
\hskip -\leftskip
#1\nobreak\hfil \nobreak\hb@xt@\@pnumwidth{\hss #2}\par
\endgroup
\fi}

Note the use of \addvspace{<space>} which inserts a vertical gap up to the <space> provided. You can change (or remove) this using the following etoolbox patch:

\documentclass{article}

\usepackage{etoolbox}
\makeatletter
\patchcmd{\l@section}% <cmd>
{}% <replace>
{}{}% <success><failure>
\makeatother

\begin{document}

\tableofcontents

\section{A section}
\subsection{A subsection}
\subsection{Another subsection}

\section{Another section}
\subsection{A subsection}
\subsection{Another subsection}

\end{document}

The same can be achieved using tocloft:

\usepackage{tocloft}
\setlength{\cftbeforesecskip}{0pt}
• great, the tocloft thing works (and it's actually neat and concise)! – Squera Dec 16 '15 at 11:46