Defending champ Buzarnescu bundled out as Vandeweghe makes winning return
Mihaela Buzarnescu's Silicon Valley Classic title defence ended at the first hurdle, while CoCo Vandeweghe made her first appearance in 10 months.
Defending champion Buzarnescu was a shock casualty on Monday, the Romanian eliminated by Daria Kasatkina 6-2 6-2 in San Jose.
Two-time grand slam semi-finalist Vandeweghe, meanwhile, enjoyed a winning return against Marie Bouzkova 6-2 6-4 at the WTA Premier tournament.
Recurring ankle and foot injuries had left former world number nine Vandeweghe sidelined since the 2018 China Open in September, however, she celebrated a successful comeback.
"It wasn't easy. To think, five months ago, I wasn't even walking to be here, playing, in my first...match of the year, I'm just full of emotions," said Vandeweghe, who also snapped a nine-match losing streak in singles, dating back to the Libema Open in June last year.
Awaiting American Vandeweghe – San Jose runner-up in 2012 and 2017 – in the next round is second seed Aryna Sabalenka.
Carla Suarez Navarro and last year's runner-up Maria Sakkari also progressed to the second round with respective wins over Magda Linette (6-3 6-3) and Ekaterina Alexandrova (6-1 6-4).
Kristie Ahn outlasted Ajla Tomljanovic 6-3 4-6 6-4 to set up a clash with third seed Elise Mertens and Mayo Hibi prevailed 6-2 3-6 7-6 (7-3) against Timea Babos.
At the Citi Open, former Wimbledon finalist Bouchard lost to American Lauren Davis 6-1 6-2 in Washington.
Next up is third seed Sofia Kenin, who was too good for Jennifer Brady 6-2 6-4 earlier on Monday.
Lesia Tsurenko – the fifth seed – eased past Wang Yafan 6-3 6-2, Christina McHale topped Anastasia Pavlyuchenkova 6-2 4-6 7-5, Nao Hibino lost 6-1 6-1 to Anna Kalinskaya, Jessica Pegula beat sixth seed Katerina Siniakova 6-4 6-3, Varvara Gracheva defeated Anna Blinkova 6-2 5-7 6-2 and Iga Swiatek saw off Ons Jabeur 4-6 6-4 6-4.