The former Getafe midfielder was brought on at half-time to replace Alexandros Tziolis and made almost an immediate impact as he slid in to divert Oscar Serrano's cross into the net.
Rayo had taken the lead in the first-half through Raul Tamudo, who took advantage of a slip by Bernardo in the home defence to run through and cleverly lob Tono Ramirez from a narrow angle.
Both sides had chances to win the match but Michu's header came back off the bar for the visitors and Adrian should have won it in the final minute - but he shot straight at Dani Gimenez.
Rayo made an extremely fast start by scoring after only 14 seconds at the Bernabeu last weekend and they began the better of the two sides in a cagey opening 45 minutes.
Neither side had mounted a serious threat before the visitors took the lead as the veteran Tamudo stayed just onside to receive Jose Movilla's ball over the top - and he calmly stroked the ball over the on-rushing Tono from the tightest of angles.
Michu should have done better with another great chance just after the half hour mark but dragged his volley at the back post into the side-netting when he had plenty of support in the box waiting for a cross.
Hector Cuper reshuffled his pack at half-time and with the introduction of Adrian at the start of the second period Racing were a much more potent attacking threat in the second-half.
In particular Pedro Munitis' influence over the game grew as the second period developed and he had a crucial part to play in the equaliser as it was his lovely floated pass over the defence that Serrano attacked at the back post - and he laid the ball across for Adrian to guide into an empty net.
Michu almost immediately had Rayo back in front but his looping header from Piti's cross came back off the bar and Pedro Botelho's claims for a penalty as he challenged for the rebound were waved away.
Pape Diop then came close to putting Racing ahead on three occasions. Firstly the Senegalese midfielder was denied by a brilliant saving challenge from Jordi inside the area before he headed inches wide a minute later. Diop then had another decent effort from long distance well saved by Dani.
Rayo's threat faded in the final quarter and Adrian had the chance to seal all three points in the second minute of stoppage-time but he could not take advantage of Manuel Arana's excellent ball through to him and Dani got down to save comfortably.
The point was enough to move Racing out of the relegation zone to 17th place whilst Rayo move onto six points and up to 14th.