Even a team performance as irrepressible and uplifting as this was not enough to separate Motherwell's players and staff from their caution.

They are within touching distance from securing a place in the Champions League qualifying rounds, but there was little air of triumph at Fir Park. Even ensuring that they will play some form of European football next season with this result was subdued by a sense of obligation.

Motherwell travel through to Tynecastle tomorrow night before playing Rangers at Ibrox on Saturday. Their final game of the campaign is then at home to Dundee United, their closest challengers for third place, which brings a Champions League spot due to Rangers' failure to gain their Uefa club licence.

Stuart McCall's side stand on the brink of a momentous achievement, and there were enough moments of intrepidness and clever attacking play in this display to confirm their status as one of the Clydesdale Bank Premier League's most consistently impressive teams this season. After a brief spell of poor form – Motherwell had picked up only one point from their previous four games – they found the means and the spirit to restore the decisiveness to their play.

"Without a doubt, that was the biggest game of the season so far because of how important the three points were to us," said Nicky Law, the Fir Park attacking midfielder. "The Champions League would not have entered anybody's mind at the start of the season, and it's huge on a personal level because how often do you get the chance to play in it? You have to play for the elite clubs, but we're still concentrating on sealing third place.

"We've had a tough run and people started to get nervous. It could be close again if we lose in midweek and Dundee United win. We won't be taking our foot off the gas; it's certainly not job done."

This was Motherwell at their most lively: slick, sure-footed, creative and prepared to play without fear. They have felt their rivals begin to press their own case for finishing third recently, but McCall challenged his players to be bold and emphatic in their play. He was rewarded with a positive and confident performance, with Chris Humphrey at the forefront of their undaunted approach.

St Johnstone, who are currently in the last of the Europa League qualifying places, could not contain the home side, in particular the right winger's pace and thrust. Humphrey won the penalty for the opening goal, that Michael Higdon converted, and set up two more goals, for Law and Jamie Murphy, before Henrik Ojamaa came off the bench to score twice in the closing 10 minutes. David McCracken's missed header, presenting a scoring chance that Murphy spurned, was typical of the visitors' forlorn display.

"We don't want to finish on a low," McCracken said. "The boys have done tremendously to get into the top six and been around third, fourth and fifth all season, but if we let the season peter out it's going to be disappointing. We want to [qualify for the Europa League] on our own merit rather than waiting for somebody else to do something."

Steve Lomas, the St Johnstone manager, disagreed with Bobby Madden's decision to award Motherwell a penalty, but acknowledged that his team defended "shockingly". The visitors' sole response was a Liam Craig penalty.

"I'll be breathalysing the players in training," said the Motherwell manager. "We don't want anybody touching alcohol because we've got too big a game coming up. It would be disrespectful to Dundee United if we thought it was all wrapped up, so we won't be taking our foot off the pedal."