The 32-year-old German, who won six times during his Grand Prix career, had a trouble-free run at the Portuguese circuit.
That prompted Mercedes motorsport director Norbert Haug to hint that if the driver was interested, then serious talks would take place about him joining the manufacturer this season.
"I don't want to raise too high hopes", said Haug. "Ralf is executing tests - and to date, nothing else has been discussed.
Season 2008
Schumacher nets DTM test
15/01/2008 AT 12:51
"Should he be inspired by the tests and the same can be said about us, then it's time to continue the discussions."
If Schumacher does join Mercedes, it will likely be to race one of their 2009-spec cars for the crack AMG squad, who have a seat available following Mika Hakkinen's retirement.
If it happens, then the DTM's most successful driver Bernd Schneider, would be his team-mate.
"From my point of view, Ralf's recent Formula 1 results didn't reflect his true competitiveness,” said the five-time champion.
"When racing for Williams, he proved that he has got what it takes to win races. Hence, he can't be that bad."
Schumacher was not the only driver in action at Estoril. Former Jaguar and Red Bull F1 racer Christian Klien, British Formula Three graduate Maro Engel, plus AMG regulars Schneider and Bruno Spengler were also there.
Season 2008
Schumacher nets DTM test
15/01/2008 AT 12:51
Season 2008
Schumacher: I'm not done yet
09/01/2008 AT 17:30