West Ham endured a tricky start to their season as they lost 2-1 to NK Domzale in a Europa League qualifier in Slovenia.
The Hammers are well placed to progress in next week’s second leg – the first competitive game at the Olympic Stadium – thanks to Mark Noble’s penalty.
But for large periods, Slaven Bilic’s side were on the back foot.
Matic Crnic put the hosts ahead from the spot and then grabbed the winner with a fine solo goal.
Relive West Ham’s defeat in Slovenia
Next week promises to be a sell-out encounter, with more than 50,000 inside the Hammers’ new home.
Noble’s penalty has given West Ham an away goal that means a 1-0 win in the home leg will take them through.
But in front of barely 3,000 in Ljubjana – the game was moved from the home of Domzale – the Premier League side faded badly after a bright first-half showing and looked well short of match sharpness.
Crnic and Noble exchanged early penalties – with Domzale’s awarded for a Havard Nordtveit handball and West Ham’s for a foul on Winston Reid.
The home side then went ahead at the start of the second half when Crnic cut in from the left and curled home.
West Ham goalkeeper Adrian had to be at his best to prevent Amedej Vetrih from adding a third with a spectacular late save.
The Hammers were without last season’s standout player Dimitri Payet, and while new signing Sofiane Feghouli enjoyed an impressive start, he barely had a touch in the second half.
And at the back Bilic looks to have problems. Michail Antonio was at fault for Crnic’s second goal and looked fragile all night, while former Leeds full-back Sam Byram looked makeshift on the left.
First-choice Aaron Cresswell is set to be out for four months with a knee injury and Bilic may need to enter the transfer market before the start of the season.
West Ham assistant manager Nikola Jurcevic:
“Our performance was not so good, and maybe I can say it was average in the first half and it was 1-1. We were not satisfied with this result and in the second half we played badly.
“I am sure we will be better next week. It will be very difficult but I’m optimistic and I hope it will be a different game in the Olympic Stadium.
“We must be better. We must win this game in our stadium because we also have big ambitions in Europe, but it will be very tough because Domzale showed they are a good team.”