MaliVai Washington, the Wimbledon runner-up who is seeded sixth, and the eighth seed, Carlos Moya of Spain, were also beaten in the second round to complete the week's elimination of all but the top two seeds - Yevgeny Kafelnikov, who beat Spain's Francisco Clavet 3-6, 6-4, 6-2, and Goran Ivanisevic.
Washington lost after a tight contest against Andrei Olhovskiy, the Russian Davis Cup player winning a final set tie-break 7-4 to take the match 7-5, 6-7, 7-6. Petr Korda, of the Czech Republic, came back from a set down to dispose of Moya 5-7, 7-6, 6-3.
Monica Seles, the top seed, made short work of her first match at the Bank of the West Classic in Oakland, California, beating Patricia Hy-Boulais of Canada 6-1, 6-2 in just 60 minutes.
Seles, twice the winner here, dominated from the start, driving forehands to the corners. She will now face Po in the quarter- finals.
Seles came into the match following a semi-final defeat to Jennifer Capriati in Chicago on Saturday. That setback dropped her from joint world No 1 with Steffi Graf to joint No 2 with Spain's Arantxa Sanchez Vicario.
"After this year is over, I just want to do better at the Grand Slams," Seles said. "Not so much the rankings. Being No 1 is definitely not a priority in my life any more. It's a bonus."
Despite a sore stomach, Seles looked sharp throughout, smacking winners from both sides. "I had no problem playing Saturday, but Sunday I had a high temperature. My stomach is still not feeling that well yet," Seles said.
Seles's ailing shoulder, which she has constantly said might require surgery, held up well. This was borne out by a 64 per cent rate on her first serve. "It feels better every day," she said.