Heather Watson blamed a “bad day at the office” as she and Laura Robson made first-round exits at the BNP Paribas Open in Indian Wells.
British number one Watson, ranked 39 in the world, was below par against Romania's Irina-Camelia Begu, losing 6-2 6-4 in one hour and 43 minutes.
"Today wasn't my best day at the office," she admitted on BBC Radio 5 live.
"I didn't really play well but my opponent is a great player and I feel like these courts really suit her game, and she played very smart. I wasn't really able to keep many balls in play. I wasn't really there today."
She added: "I feel the courts are just very different, like a clay court - they're very, very slow and it bounces very high here."
World number 43 Robson looked like she would make smooth progress to round two when she led Sweden's Sofia Arvidsson by a set and 4-1.
But the wheels came off the teenager's erratic game as Arvidsson hit back to win 2-6 7-6 (7/2) 6-1.
It is Robson's third first-round defeat on the trot after starting the year in encouraging fashion with a run to the third round of the Australian Open.
Since then, the 19-year-old has lost to the lower-ranked Daniela Hantuchova in Doha and Yulia Putintseva in Dubai.
Both Robson and Watson will now look to get back on track at the Sony Ericsson Open in Miami, which begins on March 19.