Nonlinear wave-wave interaction (WWI) in the southern South China Sea (SCS) has rarely been investigated due to sparse field observation. By analyzing datasets recorded by two adjacent moorings, we find active nonlinear WWI among diurnal internal tides (D1 ITs), near-inertial waves (NIWs), and D1-f waves in this area. Although the blue shift of NIWs can provide part of the energy for D1-f waves, the bulk of D1-f energy was generated from the nonlinear WWI. Driven by the summer monsoon, a dipole of mesoscale eddies was formed off the central Vietnam coast and intruded into the study area. The anticyclonic eddy of the dipole reduced the local effective Coriolis frequency by providing negative vorticity and suppressed the nonlinear WWI locally. During the intrusion period, D1-f waves almost vanished suggesting the important role of mesoscale eddy on adjusting the nonlinear WWI in the SCS.