Serena Williams was beaten for the first time in 2015 as Petra Kvitova booked her place in the final of the Madrid Open.
USA's Williams, the top seed, had won her last 27 matches but came up well short against Wimbledon champion Kvitova, who won 6-2 6-3 in 73 minutes.
The victory was the Czech's first against the world number one.
Svetlana Kuznetsova lies in wait in the final after she knocked out reigning champion Maria Sharapova, 6-2 6-4.
Williams' last on-court defeat came in the round-robin stage of the WTA Finals in October, where she went down 6-0 6-2 to Simona Halep.