The second seeds had not dropped a set or even been taken to an 11th game in their first three matches, but the Quarter-finals had been their ceiling at Roland Garros - together a year ago and in 2022 for Heliovaara.
A match against an unseeded pair in the last eight appeared favourable, but Balaji and Demoliner came into this on the back of knocking out sixth seeds Kevin Krawietz and Tim Puetz in straight sets.
The first seven games going with serve proved the Indian-American duo would be no pushovers, but in game eight, the breakthrough came when Demoliner's serve was routinely punished by backhand winners from Patten, taking the favourites 5-3 clear, before the Finn and Brit held to win the opening set, 6-3.
In the second set, the favourites again made a break of Demoliner's serve in the third game, battling back from 30-0 down to take it on their second break point. The Brazilian then made it an unwanted three breaks in a row from his serve in game seven, as Heliovaara and Patten continually forced them into errors to go 5-2 clear.
The former Wimbledon and Australian Open champions had barely dropped a point when serving, making a loss of serve with the finish line in sight all the more surprising. Balaji and Demoliner survived two match points and battled back, before Heliovaara handed them a lifeline with a double fault.
Balaji held serve in the next game to make it 5-4, but this time there would be no let up, as Patten rounded off a simple game win with an ace to win the match 6-3, 6-4 and reach the Semi-finals.
The 36-year-old from Helsinki and 30-year-old from Colchester will enter unchartered territory at Roland Garros against either Sander Arends and David Pel or Quentin Halys and Pierre-Hugues Herbert.
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