The Jags jetted out on Monday for a training camp in Murcia and played their first friendly of the summer against FC Pinatar last night.
Boss Alan Archibald handed a start to former Hearts star Ryan Stevenson as the forward made his debut in a red-and-yellow attacking line-up that also included Kallum Higginbotham and Steven Lawless.
But it was the hosts who took a first-half lead when Yago bundled the ball into the net after keeper Paul Gallacher made two stops.
The interval saw Archibald make a series of changes and sub Gary Fraser got the Jags back into the match with a low drive into the corner.
It didn't take long for the visitors to double their tally. Declan McDaid's shot was deflected into the net then the former Morton star completed his brace just before the end when he converted Lawless' cross to clinch the win.
Next up for Thistle is a meeting with English Championship side Brighton on Saturday before they return home to continue preparations ahead of the top-flight kick-off.
Premiership newcomers Hamilton Accies kicked off their pre- season campaign with a 1-0 win over Hearts. Grant Gillespie got the goal with a late header.
Keeper Michael McGovern was the only summer signing to get a starting berth against the Jambos while Kieran MacDonald made his bow as a half-time substitute.