The league leaders drew 3-3 at home to Deportivo Cali, with the visitors denying the Manizales hosts at the death when Frank Fabra equalised in injury time.
Once Caldas led on three separate occasions, but were in front for just 18 minutes in total as Deportivo Cali got the replies they needed in quick time.
Cesar Arias struck a first-half brace, handing the hosts the half-time lead 2-1 - scoring either side of Cali's Juan Cabezas.
Carlos Rivas made it 2-2 in the 53rd minute, although Once Caldas had looked to have won it when Juan Arango netted in the 85th minute - only for Fabra's late intervention saw a share of the spoils.
The stalemate ended Once Caldas' three-match winning run, although Flavio Torres' men remain top on goal difference from Deportes Tolima.
After being reduced to 10 when Andres Cadavid was sent off late in the first half, Millonarios's defence was beaten five times in the final 31 minutes as Atletico celebrated their first win of the phase in fine style.
Fortaleza blitzed Junior 3-1 away from home, with Juan Nunez scoring early in each half, while Deportivo Pasto's hosting of Alianza Petrolera ended 0-0.
