Less than 24 hours after stunning the Swiss top seed who was carrying a back injury, Brands himself was sent home by Hanescu, 2008 champion on the high-altitude clay of this alpine village.
Brands took treatment on court three games into the second set on the way to the loss in 70 minutes.
Hanescu, ranked 54th to the 55 of Brands, won his tenth match at Gstaad, finishing with five aces and a break of serve per set.
"I'm very happy to play this event, it's of course a favourite of mine," said the winner.
"I was pleased with how I played against a tough opponent. Brands doesn't give you anything out there and I'm just glad I could reach another semi."
The Romanian next takes on sixth seed Mikhail Youzhny, a winner over fourth-seeded tournament debutant Juan Monaco 5-7, 6-4, 7-6 (9/7).
Monaco was playing his sixth quarter-final of the season, but was unable to stop Youzhny from reaching a second Gstaad semi-final in three years.
The number 33 Russian's most recent success was a runner-up spot against Federer on the grass of Halle, Germany, last month.
The 31-year-old Youzhny won his 23rd match of 2013 and his tenth on clay at a venue where the surface plays more like a hardcourt due to its location 800 metres above sea level. The victory took just over two and a half hours.
With crowds understandably down after the shock exit of 2004 winner Federer, Stanislas Wawrinka was hoping to take up the slack as he played for a semi-final spot against Spaniard Feliciano Lopez.
The tenth-ranked Swiss is competing in Gstaad for the tenth time, with a 2005 final his best showing.
The other quarter-final pits Dutchman Robin Haase against Spain's Marcel Granollers.
Haase has a decent record on mountain clay and will be defending the Kitzbuehel title next week in Austria.
Results from day five of the ATP Swiss Open in Gstaad (x denotes seeding):
Victor Hanescu (ROM) bt Daniel Brands (GER) 6-3, 6-4
Mikhail Youzhny (RUS x6) bt Juan Monaco (ARG x4) 5-7, 6-4, 7-6 (9/7)
Robin Haase (NED) bt Marcel Granollers (ESP) 5-7, 6-3, 7-6 (7/5)