Ninth seed Alison Riske defeated former top 10 player CoCo Vanderweghe in the opening round of the Silicon Valley Classic in straight sets on Monday.
Riske, who made the Wimbledon quarter-finals in 2019 and is currently ranked 37th in the world, won 6-3 6-4 in one hour and 28 minutes.
In a match which saw eight breaks converted, Riske's ability to win on her first serve proved the difference.
Vanderweghe, who made the Silicon Valley Classic finals in 2012 and 2017, was broken five times and won 58.1 per cent of first serve points.
Croatian sixth seed Petra Martic eased into the second round with a 6-3 6-3 victory over Anastasija Sevastova.
Martic will face Magda Linette next, after the Pole accounted for Dutch qualifier Lesley Kerkhove 6-3 6-3.
Ana Konjuh defeated Marie Bouzkova in a three-set epic which last two hours and 41 minutes, winning 7-5 3-6 7-5.
China's Zhang Shuai won over Emma Raducanu in straight sets, along with Carolina Garcia who defeated Emina Bektas.