The 62-year-old was left bitterly disappointed following the world and European champions' slip-up to South Africa, a 1-0 defeat at the scene of their 2010 triumph
The world and European champions suffered a surprise 1-0 reverse at Soccer City in Johannesburg, the scene of their World Cup 2010 final triumph over Netherlands.
The result follows a narrow victory over Equatorial Guinea on Saturday and Del Bosque concedes that La Roja have been well below average.
"It could be [our worst recent performance]," he told Cuatro. "I think we can even say the defeat was well deserved.
"We started well but little by little they started to open us up and we allowed them time to counterattack.
"We came out with good intentions and fought to the end to find an equaliser so I can't complain about the players' attitude. [South Africa] got better as the game went on, though."
Bernard Parker scored the only goal of the encounter in the 56th minute, dinking over Spain goalkeeper Victor Valdes, who was forced off with a serious-looking knee injury later in the match.
Del Bosque's men nearly mustered an equaliser, with Raul Albiol only denied by an outstanding Itumeleng Khune save late on, but South Africa held on for a famous win.
The loss was Spain's first since the Confederations Cup final, in which they were defeated 3-0 by hosts Brazil.