A 7 December rematch in Saudi Arabia was announced on Friday and Joshua’s promoter Eddie Hearn told a a news conference on Monday it was signed.
Ruiz did not speak or post about the bout on social media until Wednesday.
The 29-year-old told ESPN in Mexico: “I have not signed anything yet, we are negotiating everything.”
Asked about a Saudi Arabia contest he said: “They want to do it there but we have to see where we negotiate with my team. I would like it in New York again.”
Ruiz stunned Joshua – who was a 1-25 favourite with bookmakers – at New York’s Madison Square Garden on 1 June.
In doing so, he walked away with the IBF, WBA and WBO heavyweight titles, becoming Mexico’s first world champion in the division.
Joshua, 29, took up his right to a rematch and promoter Hearn has repeatedly pointed to Ruiz being signed to fight as a result of the fact a rematch clause was part of the first fight’s contract.
Hearn has said Joshua’s team contractually only need to let Ruiz “know the time, date and venue” and says the champion could face legal proceedings if the fight does not happen.
Organisers in Saudi Arabia have put forward a reported fee of $40m (£33m) to host the event but there has been criticism of taking a fight of such magnitude there given the country’s human rights record.
Human rights organisation Amnesty International has said the move provides “opportunity for the Saudi authorities to try to ‘sportswash’ their severely tarnished image”.
Ruiz is again a bookmakers’ underdog for the repeat fixture but since his shock seventh-round stoppage win has stated Joshua is “not good at boxing”.