South capitalised on the withdrawal of Serena Williams
Melanie South's good run at the East West Bank Classic in Los Angeles ended with defeat by China's Meng Yuan.
The British number four, ranked 156 in the world, made the main draw as a lucky loser and beat world number 66 Petra Kvitova on Wednesday.
But despite a late rally she could not prevent a 6-2 6-3 defeat by Meng, the world number 122.
Top seed Jelena Jankovic remained on course to become the world's number one by beating Melinda Czink 6-3 6-2.
Victory at this event will allow the 23-year-old Serb to leapfrog her compatriot Ana Ivanovic in the world rankings.
Playing her first tournament since injuring her knee at Wimbledon last month, Jankovic dominated her left-handed Hungarian opponent, controlling the rallies and sending down seven aces in a powerful display of serving.
"If I could hit this serve every day like that, my life would be so much easier," said Jankovic.
"You don't have to break a sweat and you're winning games. There's no pain in my knee and I'm really happy about that."
Jankovic will face Nadia Petrova, who beat fellow Russian and fith seed Vera Zvonareva 6-4 7-5.
Third seed Anna Chakvetadze was also beaten, losing 6-4 5-7 6-2 to Austria's Sybille Bammer.