The second-seeded Russian broke Diaz-Oliva's first service game in the first set to take a 3-0 lead. She broke again in the seventh game of the second set.
Petrova, who made it to the fourth round of Wimbledon last year, is the highest remaining seeded player after Elena Dementieva retired in her opening match Tuesday.
Also, Michaella Krajicek upset third-seeded Anabel Medina Garrigues 6-3, 3-6, 6-2. The 16-year-old Krajicek dominated the deciding set, breaking the Spaniard twice and gaining break points in each of Medina Garrigues' other service games.
The fourth-seeded Safina won the last four points of the second-set tiebreaker to beat Maria Kirilenko 6-4, 7-6 (2).
In other matches, Gisela Dulko beat Maria Elena Camerin 6-3, 6-0, Klara Koukalova defeated Claudine Schaul 4-6, 6-4, 6-0; Lucie Safarova beat Virginia Ruano Pascual 6-1, 6-1; and Meghann Shaughnessy beat seventh-seed Anna-Lena Groenefeld, who retired in the third set while trailing 7-6 (9), 3-6, 3-0.
Roger Federer and Andy Roddick were seeded 1-2 for Wimbledon on Wednesday, setting up a possible rematch of last year's final.
Federer, a two-time defending champion, got the top seeding in line with his world No. 1 ranking.
Roddick was seeded two spots above his ATP ranking, ahead of 2002 champion Lleyton Hewitt of Australia and French Open champion Rafael Nadal.
Hewitt was seeded No. 3, with Nadal -- who has little experience on grass -- at No. 4.
Federer beat Roddick 4-6, 7-5, 7-6 (3), 6-4 in last year's final. He has four Grand Slam titles. Roddick won the 2003 U.S. Open.
Among the women, Lindsay Davenport was seeded No. 1, followed by defending champion Maria Sharapova, Amelie Mauresmo and former two-time champion Serena Williams.
Last weekend, Federer won the Wimbledon grass-court tuneup at Halle, Germany, for the third straight year. He's won 29 straight matches on grass; his last loss on the surface was in June 2002 against Mario Ancic in the first round at Wimbledon.
Roddick won the Queen's Club title in London for the third consecutive year on Sunday. He said he thought Hewitt would get the No. 2 seeding.
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