St Johnstone 0 Partick Thistle 3
Edwards curled home Thistle's second goal in a 3-0 victory over St Johnstone to make it five wins in six trips to Perth for Alan Archibald's side.
Steven Lawless had opened the scoring from the penalty spot five minutes into the second half after Steven Anderson was ruled to have brought down Kevin Nisbet, and Chris Erskine finished the job in the last minute.
Nisbet was handed his second start for Thistle after Kris Doolan picked up a hamstring injury and the 20-year-old showed his youthful innocence early in the first half when he stayed on his feet after having his ankle clipped as he turned inside Anderson. The striker had a shot blocked but had been knocked off his stride by the challenge and referee Euan Anderson played on.
Nisbet got the only shot on target of the first half after robbing Joe Shaughnessy and showing good feet before his near-post effort was blocked by Alan Mannus.
The visitors were again without the injured Blair Spittal and they lost Danny Devine on the half-hour mark after he failed to run off an early knock. Niall Keown came on to partner debutant Jordan Turnbull in central defence.
After an uneventful first half, it was no surprise that the breakthrough came from a set-piece - but the penalty award shocked the hosts as Anderson claimed he got the ball as he and Nisbet chased Erskine's forward pass. The striker was going away from goal just inside the box and Lawless took full advantage of the unexpected chance by sending Mannus the wrong way.
Anderson quickly had a header cleared off the line by Erskine before Nisbet twice curled shots wide from just outside the box.
Ryan Edwards helped Partick Thistle into the Betfred Cup quarter-finals with a brilliant strike at McDiarmid Park.
Saints skipper Anderson's nightmare continued when he somehow sent a free header wide from six yards following Wotherspoon's cross.
And Edwards lit up the game in the 62nd minute when he curled into the top corner after a neat exchange of passes just outside the Saints box.
The hosts had a chance to get back into it when Steven MacLean threaded Graham Cummins through but the substitute lacked composure and sliced wide from 18 yards.
Saints continued to press but it was evident it was not to be their night when Shaughnessy failed to beat Tomas Cerny from six yards - and that was confirmed when the referee waved away penalty appeals as Cummins went down under the aerial challenge of Keown.
And Erskine soon raced away to finish in style after an excellent flick-on by Nisbet