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.
|Serena's winning sequence comes to an end|
|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. Although technically unbeaten in 2015, Williams did withdraw before her semi-final against Halep at Indian Wells in March due to a knee injury.|
Williams will bid for her 20th career Grand Slam title when the French Open begins later this month.
She indicated she was not too disappointed to lose her unbeaten record with the second Grand Slam of the year looming.
"I'm just exhausted today,'' said Williams, the Madrid champion in 2012 and 2013.
"But it's really good preparation for Roland Garros.''
Unseeded Kuznetsova beat fellow Russian Sharapova for the first time in seven years, breaking her opponent's serve three times and saving the one break point she faced.
"I'm just happy to be in the final," said Kuznetsova, a two-time Grand Slam winner but currently down at 29 in the world.
Sharapova said: "Today she had much more pop on her ball than I did, while I felt my shots were a bit too flat.
"I also think the court and the bounce, with the altitude here, meant her spin was getting really high."