Cillian Sheridan opened the scoring last night as St Johnstone hammered Rangers 4-1.
It was the Celtic striker's fourth goal since moving on loan to the Perth side after a disappointing time at Plymouth in the first half of the season.
The defeat leaves Rangers 10 points clear of Celtic at the top of the SPL with the Ibrox men having a game in hand.
St Johnstone raced into a 3-1 lead before half-time with manager Derek McInnes thrilled by his side's display.
He said: "I've been delighted with the players' response and attitude towards playing in this league. We deserved that and we're pleased to send our supporters home happy. I hope we can use it to finish the season strongly."
Walter Smith may take pride in the winning mentality he's instilled at Rangers but admits having no complaints about Saints taking the points.
The Scottish champions found themselves trailing to goals from Sheridan and Chris Millar in the opening 12 minutes as the Saints got off to a blistering start in the rearranged fixture at McDiarmid Park.
And Gers' hopes of a comeback when Sasa Papac pulled a goal back proved premature when the home side struck again either side of the break through Liam Craig and Murray Davidson.
Smith said: "You get disappointed when you lose games. It was a very disappointing night. Overall, I can't begrudge St Johnstone their win because they played very well."
Saints' priority at the start of the season was surviving their first campaign back in the top flight but they are now real contenders for a top-six finish.
Meanwhile, former Manchester City assistant manager Mark Bowen has admitted that managing Celtic would appeal to Mark Hughes.
Hughes won't be short of offers to return to the Premier League but Bowen, who has worked with Hughes at Wales, Blackburn and City, refused to write off a move to the SPL.
He said: "Mark has always said it has to be the right vehicle, the right situation. He's always been a manager or coach who wants to go in and build a place and make a difference long term.
"It's what we did with Wales and Blackburn and we were in the process of doing at Manchester City.
"It seems to be the natural thing at the moment that Mark is linked with any vacant position. His talents are widely recognised and since City we've been waiting for the right opportunity.
"In that sense, I suppose we were expecting this speculation and it's a pleasant expectation to see that he's been linked with the Celtic post. Mark's credentials are there for all to see."