Both parties are also deep in negotiations to find a leader who would appease their voters, with Mattarella having a final say. "Good morning friends!", Salvini said via Twitter in a post featuring a photo of a cup of cappuccino on Tuesday.
Mr Salvini's appointment with Mr Mattarella was set for early Monday evening.
Di Maio and Salvini spoke after separate meetings in Rome with President Sergio Mattarella, which many hoped could lead to a breakthrough and the announcement of a compromise candidate for the premiership.
"We agree we have to move quickly, but as we are writing what will be the government programme for the next five years, it's very important for us to do it as well as possible, so we told the president we needed a few more days", Di Maio said.
The 5-Star Movement has already said it would put any accord to an online vote of its own members.
The best choice of Italy's anti-establishment 5 star Movement claims he's agreed on the coalition programme with all the rightwing League. "I'm proud that we are discussing, even heatedly, about the kind of Italy we want to create", said Salvini, adding there were differences on issues including immigration, justice reform and infrastructure.
After their separate meetings with Mattarella, Di Maio and Salvini denied that disagreements over who should be premier were to blame for the delay in reaching a coalition pact. Salvini said the League will hold a similar consultation with its electoral base, organizing an informal referendum in large Italian squares this weekend.
Italy has been in political deadlock since an inconclusive March 4 election, which was dominated by concerns over a struggling economy, the refugee crisis and illegal immigration.
They have also both said they want to override European Union budget deficit restrictions to help boost growth and are opposed to economic sanctions on Russian Federation, which they have pledged to try to overturn.
The 5Stars and the League have been negotiating a government deal since last week, when Salvini's election partner, former Prime Minister Silvio Berlusconi, gave the green light for the two populist forces to form a government without his Forza Italia party.
Their flagship economic measures are also seen as too ambitious and expensive for a heavily indebted country like Italy, which has the second-largest debt in the eurozone after Greece.