Watson looked competitive in the opening set but lost a key fifth gameBritons Heather Watson and Katie Boulter fell to straight-set defeats in the first round at Indian Wells.
World number 113 Watson, 29, battled early in set one but failed to take two break points at 2-2 and Czech Tereza Martincova pushed on to a 6-2 6-1 win.
Boulter, 25, was broken three times in her opening set against Italy's Jasmine Paolini and lost 6-3 6-2.
British interest in the tournament rests on Heather Dart and Emma Raducanu, who are both into round two.
However, British qualifiers Watson and Boulter both struggled to gain a footing in testing contests played out in blustery conditions in California.
Martincova simply proved too strong for Watson, winning 62.8% of points on her first serve, compared to the 43.3% achieved by her rival.
The 27-year-old - who is world number 42 - will now play defending champion Paula Badosa of Spain.
Paolini took all five of her break points on the day and eased to victory, despite giving up her own serve twice in the opening set.
The world number 46 will now face number-two-seed Aryna Sabalenka in round two.
