O'Brien is looking to improve on her current world ranking of 92
Britain's Katie O'Brien was beaten 6-2 6-1 by Bethanie Mattek-Sands in the first round of the Cellular South Cup.
The British number three, 92nd in the world, rarely threatened and was broken five times by the American wildcard in a match that lasted only 55 minutes.
O'Brien won 12 points in her opponent's seven service games in Memphis.
Britain's number one Elena Baltacha, world ranked 70 after her Michigan title win, plays Renata Voracova from the Czech Republic, ranked 115.
In January, O'Brien, 23, reached the second round of a Grand Slam tournament for the second time before losing to Jelena Jankovic at the Australian Open.
Russian top seed Maria Sharapova is in action on Monday against American Shenay Perry.