Celtic manager Brendan Rodgers insisted St Mirren had provided the Hoops' biggest domestic test this season after Celtic's 4-1 Scottish Cup win over the Buddies.
Jack Ross' Saints stunned Celtic Park by taking the lead through Harry Davis in the 13th minute. They held on until Mikael Lustig headed the equaliser in the 58th minute and goals from Scott Sinclair, Moussa Dembele and Leigh Griffiths booked Celtic's place in the semi-final.
Rodgers' men have swept all before them in Scotland this season having a near-perfect league record of 26 wins and one draw and the League Cup already in the Celtic Park trophy cabinet.
The former Liverpool boss heaped praise on the Buddies after the tie and said that the Championship's bottom side had been the toughest Scottish team Celtic had faced, and not Premiership big guns such as Rangers, Aberdeen and Hearts.
He said:"I have to give credit to St Mirren, I think they are the best team we have played domestically including Premiership teams.
"Jack has done an excellent job. They are very well coached and they made some signings in January which will have helped him. How they are bottom of the Championship I do not know.
"They are very well organised, they have a good idea of the game, they play with composure and they have speed and composure in their team.
"They are a very, very good side."
Rodgers reserved special praise for opposing manager Ross,saying he sees something special in the former Alloa manager.
He said:n"We played Alloa here so I have an idea of his coaching abilities. He's a very good coach.
"There's a couple of moments in games that make me think of the level of coach. It's just an extra little detail that can define coaches and I seen that in his team today."