Saints had netted 12 times in their last two games, scoring seven without reply in their Betfred Cup victory over Brechin City and seeing off Hamilton 5-3 last weekend.
However, despite dominating at McDiarmid Park, they could not find a way to break down a resolute United defence.
Marc McNulty, making his first start since joining on loan from Reading, came close to an early opener for United.
The striker, part of a three-man forward line for the visitors, initially mis-kicked a cross from Lawrence Shankland but recovered quickly to fire in a fierce shot that slipped just over the bar.
United enjoyed brief periods of possession thereafter, but all the chances fell for Saints.
Then four minutes from the interval, the Perth outfit again came close to the breakthrough. McNulty was guilty of a loose pass at the edge of his own box and Craig Bryson whistled a drive inches past.
The pattern was repeated after the break as Saints continued to carve out all the opportunities.
Not everything was coming off for Wotherspoon, who was wasteful at times, but the former Hibernian player was proving the home side's best outlet in attack.
On the hour mark, the 30-year-old drove another effort over the crossbar before coming closer with 18 minutes remaining.
A break of the ball 40 yards from goal allowed Wotherspoon to burst forward at pace. A partial block by the backtracking Mark Connolly took the sting out of the shot and Siegrist dived full length to his right to claw the ball away.
And the United keeper came to his side's rescue again in the final minute.
This time, Wotherspoon was the provider with a corner that Murray Davidson bulleted towards goal, but Siegrist blocked the header on his line to seal a point for the Tangerines.