Four minutes into injury-time, Andreu found the bottom left-hand corner of the net from outside the box to grab a point for Hamilton who slip to second in the Scottish Premiership table.
Andreu’s late goal also denied Partick a first away win of the season after James Craigen’s quickfire double appeared to have earned them three points.
The Accies were seemingly heading towards an easy victory after dominating the early stages and moving 2-0 ahead inside 24 minutes.
But, after pulling a goal back thanks to a touch of good fortune, Thistle took full advantage with Craigen on target twice within the space of four minutes in the second half.
After Mickael Antoine-Curier and Daniel Redmond had gone close for the hosts, Hamilton deservedly took the lead on 22 when Dougie Imrie laid the ball to Darian MacKinnon who located the bottom left-hand corner of the net from just outside the box.
And 90 seconds later it was 2-0 as Antoine-Curier lobbed his assist to Redmond who lashed the ball past Scott Fox from 10 yards.
Somehow, the visitors pulled a goal back on a rare break in the 32nd minute through Jordan McMillan whose half-volley from the edge of the box took a sizeable deflection off Ali Crawford before beating Accies keeper Michael McGovern.
It was the first time in 442 minutes that Hamilton had conceded a goal on home turf.
As the hosts attempted to extend their lead, Fox saved well from Crawford and Andreu before Thistle snatched an equaliser through Craigen.
With 67 minutes played, Craigen levelled matters with a clinical strike inside the box after being set up by Steven Lawless.
And the midfielder fired in Partick’s third, thanks to Christie Elliott’s assist, with a half-volley that found the top right-hand corner.
But with Partick on the verge of a shock win, Andreu came up with the sixth goal of the game at New Douglas Park spectacularly firing home from 25 yards.