Node Voltage Analysis (diamond like circuit)
A method that also gives good result is redrawing the sketch in a simpler form as shown below:



Current \$i_3\$ is the sum of \$i_2\$ and \$i_1\$ and equals:

\$\frac{10-5}{2R}=\frac{5}{R}+i_1\$

Solving for \$i_1\$ gives \$i_1=-\frac{2.5}{R}\$. As pointed out by jonk, considering the negative sign, the 5-V generator sources \$i_1\$.