Novak Djokovic’s 22-match winning steak ended at the hands of an inspired Juan Martin del Potro, who beat the Serbian star 4-6 6-4 6-4 to reach the Indian Wells Masters final.
Djokovic had not lost a match since falling to Sam Querrey at the Paris Masters on October 31.
His 17 wins this year included a run to a fourth Australian Open title and a triumph in Dubai.
In the final, Argentine seventh seed del Potro will face 11-time grand slam champion Rafael Nadal.
Nadal earlier on Saturday beat Tomas Berdych 6-4 7-5 in his semi-final.
Nadal has now reached the final in four straight tournaments since returning last month from a seven-month injury absence that followed his shock second-round exit at Wimbledon last year.
“For me, it’s completely unexpected,” Nadal said.
“To be in the final … it’s a big surprise but I’m here and I’m very, very happy.”
The fifth-seeded Spaniard saved three break points in the final game against sixth-seeded Berdych before gaining a first match point.
A wayward forehand cost him that one, but Berdych’s backhand into the net gave him another, and when the Czech knocked a backhand service return into the net Nadal thrust his arms into the air in triumph.
Nadal booked his first hard-court final since the 2012 Australian Open, when he was beaten by Djokovic in five sets.
The Spaniard hadn’t even entered a hard court event in nearly a year, since withdrawing from the Miami Masters last year with tendinitis in his left knee.
That same troublesome knee would later lead to his lengthy absence from competition.
Nadal launched his comeback with three modest clay court tournaments in Latin America, reaching the final at Vina del Mar and winning titles in Sao Paulo and Acapulco.
But he has faced his sternest test yet in the first Masters tournament of the season, not only from the more punishing court surface but as part of a field that featured 49 of the top 50 men in the world.