Venus Williams fell to teenager Anastasia Pavlyuchenkova 3-6, 6-1, 6-4, the second time in a week the Russian has beaten her. Novak Djokovic had 10 aces in a 6-3, 7-5 win over Romania's Victor Hanescu to reach the second round.
Safina would lose her top ranking if she finishes behind No. 2 Serena Williams in this tournament. Williams won her first-round match Saturday against Estonia's Kaia Kanepi and next faces Russia's Ekaterina Makarova on Tuesday. Safina can hold the top ranking if Williams loses.
The latest defeat comes a week after Safina was knocked out of the second round of the Toray Pan Pacific Open by 18-year-old qualifier Chang Kai-chen of Taiwan.
"I woke up this morning feeling really excited, but didn't do any special preparation and never expected to win the match," said Zhang, ranked 226th.
Safina made 13 double-faults and began shouting and growing distraught toward the end of the match. She did not speak to reporters.
On Tuesday, No. 2 Rafael Nadal and defending champion Andy Roddick begin play in the men's draw.
Venus Williams got off to a strong start against an 18-year-old opponent who defeated her in the second round in Tokyo last week. Serving for the match, Pavlyuchenkova sent a forehand winner past Williams to wrap up the final set.
"She played really well. Sometimes I just made errors too soon in the point," said Williams, who totaled 12 double-faults.
Williams added: "She plays well against everyone she plays and I think I usually do, too."
Pavlyuchenkova said she'd learned from her defeat to Williams this year in Dubai.
"I just tried to play as simple as I can just into the court and just make her move," she said. "And today I just tried to do the same thing."
"She started actually way aggressive today especially the first set. She was playing much better. She was already preparing for this match more. ... She wanted to kill me I guess," she added with a smile.
Djokovic, seeded second and ranked No. 4, won despite inventive play at the net by the 27th-ranked Hanescu.
In the first set, Djokovic was up 4-2 when he saved two break points to hold serve and remain in control. The second set began with the players trading breaks to two games apiece. Each held serve until Djokovic broke Hanescu to go up 6-5 and served out for the match.
"I was happy overall with the way I played. It's just that I thought I could finish the job earlier. It was a little bit frustrating the way I was losing some points in the second set," Djokovic said. "That's the first round. You have to get used to the ups and downs a little bit and hopefully the next match will be more consistent and I'll be able to play my best."