A late tap-in from Michael Gregoritsch gave Freiburg a 1-0 victory against West Ham in the first leg of their Europa League last 16 tie on Thursday.
After a testy opening, Conference League holders West Ham almost took the lead on the 65-minute mark, but Freiburg goalkeeper Noah Atubolu denied Jarrod Bowen with a superb diving save.
Gregoritsch scored just 11 minutes after coming on, tapping in from a metre out, his fifth goal in seven Europa League games.
"We played badly today," admitted West Ham captain Kurt Zouma.
A "very proud" Freiburg captain Christian Guenter told RTL "we threw everything at them – I think we deserved to win."
West Ham appealed for a penalty for handball deep into stoppage time after Noah Weisshaupt touched a Kalvin Phillips shot, sending the referee to the monitor.
Freiburg survived a long VAR review to hang on to a slender lead ahead of next week's second leg in London.
"I don't know if it was the right decision. The decision took a bit of time and the referee took a lot of time. It is what it is. The game is over. I don't want to go back to it now," added Zouma.
"We were a little lucky today," said goalscorer Gregoritsch. "If a review goes on so long, you have to think to yourself it can't be that clean cut."
Hoping to make the quarterfinals for the first time, Freiburg came into the game having held Bayern Munich to a 2-2 draw at the same venue on Friday.
The teams had already clashed in the group stages, with the English side winning on both occasions.
The hosts started strongly and almost had the lead after 30 seconds. West Ham defender Vladimir Coufal got a last-ditch block on Roland Sallai's attempt, setting the tone for a strong opening period for the hosts.
The visitors had the better of much of the second period, with Konstantinos Mavropanos almost heading past some sleepy Freiburg defending at the far post on the 52-minute mark.
Atubolu leapt to deny Bowen on the counter after 65 minutes, the 'keeper getting an outstretched hand on the England winger's shot.
Freiburg manager Christian Streich, now in his 12th year as head coach after a playing career at the club, brought on striker Gregoritsch in the 70th minute.
The move soon paid dividends, with the Austrian forward in the right place at the right time to tap in a low shot from Sallai with nine minutes remaining.