Second-half goals from Danny Swanson and David Wotherspoon, and a red card for striker Greg Stewart, saw Dundee well beaten, leaving Hartley to lament his side’s lack of energy.
“That was the worst performance of our season by far,” he said. “We had no no spark, energy or tempo to our play.
“The players looked really leggy, which is not like them because they’ve been terrific apart from this.
“Overall it’s been as bad as we’ve been and I don’t want to use Wednesday’s game against Celtic as an excuse. It was just one of those days.
“They have been few and far between this season to be fair. But we don’t want the season to just fizzle out.
“We have to pick ourselves up for the next match, which is on Friday, and we’ll go from there. We’re disappointed, obviously, but we’ve got to get over it and only we can do that.”
Unsurprisingly, St Johnstone manager Tommy Wright had nothing but praise for his own players, who moved above Dundee United into fourth place in the Scottish Premiership.
Wright said: “I’m already proud of them to get top-six again.
“But we’ve still got a lot to play for and our aim now will be to get away from Dundee United and, if we can catch Inverness, that would be brilliant.
“To make it into Europe two seasons in a row would be a fantastic achievement for this club. I’m really just delighted. We scored two good goals and deserved to win.
“This will give us confidence for the run-in. We’ve got some big games coming up and we’ll just take them one at a time.
“I’ve got confidence in the players to keep doing well and they have got to keep working hard, which I’m sure they will do.”