Both nations won their first ties earlier in the mixed teams tournament, but the Spanish pairing of Tommy Robredo and Maria Jose Martinez Sanchez, aiming to give their country its third title, remained unbeaten when they were handed the 3-0 victory over Romanians Sorana Cirstea and Hanescu.
The Spanish only need to win one of their three rubbers against Australia on Thursday to be certain of reaching Saturday's final.
Martinez Sanchez was too smart for Cirstea in the women's singles, using deft ball placement and touch rather than power to unsettle the Romanian, winning 6-4, 6-3 in 92 minutes.
Robredo, who was part of the last Spanish team to win the Hopman Cup when he teamed with Arantxa Sanchez Vicario in 2002, then took the first set off Hanescu, 6-3, before the Romanian retired.
Hanescu, who struggled with cramp against Lleyton Hewitt on Saturday, sought treatment after the third game for what appeared to be an upper leg or groin injury.
He played on but conceded as soon as Robredo claimed the first set, forcing the Romanians to also concede what would have been a dead mixed doubles rubber.
Regardless of whether Hanescu is fit for Thursday's clash with the United States, the clean sweep means Romania cannot reach the final.
Earlier, Australia kept their final hopes alive with a 2-1 win over the USA.
The top seeds were stunned by Romania on the opening day of the mixed teams tournament, but bounced back to trounce the Americans, who are now out of contention after also losing to Spain on Sunday.
Samantha Stosur, the world number 13, rebounded after her surprise loss to Cirstea on Saturday to fire down 14 aces and easily account for Melanie Oudin in straight sets, 6-2, 6-4.
Hewitt then secured the tie for Australia with a barnstorming start, winning the first five games of the match on his way to an impressive 6-1, 7-5 win over lanky American John Isner.
The Americans gained some consolation by winning the dead mixed doubles rubber in a match tiebreak.
Hewitt, aiming for a return to the top 10, made it two wins from as many singles matches to start 2010, dismantling the imposing Isner serve early.
The former world number one was delighted with his form against Isner, particularly with the way he handled the big man's serve, breaking it three times despite 13 aces.
"It was important to get a good start and jump him early and I was able to do that," Hewitt said.
"I felt like I was reading his serve very well.
"I moved extremely well, I passed extremely well and served as well as I have in a long time."
Hewitt said strapping on his knee was simply precautionary, after slightly jarring the joint on Saturday.