In a sparsely populated Gamla Ullevi stadium, the visitors settled into the match the quicker. Yet, despite chasing shadows in the first few minutes, the hosts had the first real chance, but as Rayo keeper Augusto Batalla raced from his goal, Adrian Svanback inexplicably slotted wide.
Isak Brusberg was next to break forward, with only a last-ditch tackle keeping the scores goalless, before a three-on-one was also passed up when Pontus Dahbo opted to delay in possession rather than pass to an unmarked Brusberg.
The hosts were made to pay for their wastefulness less than a minute later as Rayo went straight down the other end and took the lead through Alvaro García, who slotted home after a fine Sergio Camello cross.
Undeterred, Brusberg missed a fourth chance in the opening 16 minutes alone for Hacken when one-on-one with Batalla.
Rayo sat on their lead, but as HT approached, Julius Lindberg gave the home support something to shout about as he finally rolled in an equaliser - his first goal since late August.
That came in a 2-1 defeat, and the hosts may well have gone into HT aggrieved that Rayo’s Oscar Trejo wasn’t sent off for an overly physical challenge having already been booked.
A disjointed start to the second half saw much of the play taking place in a congested midfield that neither side could take control of. With the pitch cutting up badly, players began to lose their feet, and the conditions seemed to suit the hosts.
Svanback would’ve added a second were it not for Jozhua Vertrouwd’s intervention, however, from the next Hacken attack, Lindberg’s brilliant cross was poked home on the slide by Brusberg to complete the turnaround.
Just after the hour, Rayo’s triple substitution signalled their intent, whilst the hosts were forced into a change of their own with Silas Anderson unable to continue.
Pacha Espino’s speculative long-range effort came back off the woodwork as the Red Sashes pressed once more, with gaps beginning to form in the final 10 minutes.
Samuel Leach Holm’s venomous free-kick had Batalla at full stretch to keep it out, while Rayo’s Jorge de Frutos smashed a volley agonisingly wide.
De Frutos then went over in the area and it appeared the visitors would be offered a lifeline from the spot. However, the official was well-placed and waved away the protests.
In the latter stages of stoppage time, Rayo were eventually awarded a penalty after a VAR review, and, eventually, Ratiu tucked home seven minutes after the allotted time was up to preserve Rayo’s unbeaten run in the competition.