Alison Riske won to offset Madison Keys' shocking loss for the American fans at the Bank of the West Classic.
Madison Keys suffered a lapse at the Bank of the West Classic on Wednesday, the seventh seed losing to Ajla Tomljanovic.
Prior to arriving in Stanford, Tomljanovic had not come from a set down to win a match since the Australian Open, but she has now done so in each of her outings at the WTA Premier hard-court event - prevailing 1-6 6-4 6-1 on this occasion.
Keys won the first set in just 23 minutes, but surrendered the third to Tomljanovic in 25 - as the Croatian, and soon-to-be Australian, representative completed another comeback.
It was the 18th-ranked Keys' first outing since her run to the Wimbledon quarter-finals, a tournament she has now bookended with first-up losses in a sign of her inconsistency.
Alison Riske downed a top-10 opponent for the second time in her career, dumping out third seed Carla Suarez Navarro 6-4 6-4.
Riske previously beat Petra Kvitova in the US Open third round in 2013, and came from 3-1 down in the first set to see off world number 10 Suarez Navarro.
World number 59 Riske said: "I felt like we both played pretty well in the first set - it was close. I went up big in the second, and I feel like I got a little bit tentative, but I'm happy that I got myself together and closed the match out."
Mona Barthel led by a break of serve in each set, but needed a tie-break in the decider to defeat sixth seed Andrea Petkovic 5-7 6-2 7-6 (7-4).
Eighth seed Elina Svitolina was the only seeded player to win through her second-round match on Wednesday, downing qualifier Nicole Gibbs 6-3 7-6 (7-5).