Di Maio said both he and Salvini "agree on the fact that we won't publicly name names".
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.
The two parties are looking to implement massive tax cuts, abolish unpopular pension reform and introduce new welfare payments to Italy's growing army of poor.
But the biggest single bloc, at some 37 percent, is a center-right alliance with Salvini's League and Berlusconi's Forza Italia party.
According to reports, the parties have agreed on rolling back increases to the age of retirement, while the M5S is willing to follow the League's hardline anti-immigration policies.
Tim Allen's Last Man Standing Renewed For Season 7 At Fox
Its ratings weren't too shabby either; Last Man Standing averaged a 1.7 rating in the key 18-49 age demographic. Fridays - the same time slot it occupied for five of six seasons before it was canceled last spring by ABC.
Five Star has also called for a universal basic income scheme for the unemployed, while the League supports a flat income tax.
But it seems the decision has come too late, as the M5S and the League have reportedly come to an agreement on a government program. However, Mattarella warned over the weekend that he would not be a "pushover".
The forerunner of Salvini's League, the Northern League, was the main partner in the three center-right governments headed by media mogul Silvio Berlusconi starting in 1994.
The 5-Star Movement has already said it would put any accord to an online vote of its own members.
Berlusconi gave "effective and constant proof of good conduct" after carrying out his punishment, Italian news agency ANSA quoted the tribunal judges as concluding.
"To think that we can get by alone is a pure illusion or, worse, a deliberate deception aimed to sway public opinion", Mattarella said at State of the Union conference in Florence on Thursday.