Also winning their second-round matches on Wednesday were the second seed Lindsay Davenport, the fifth seed Brenda Schultz-McCarthy of the Netherlands, Kimberly Po and Linda Wild.
Seles, twice the winner here, dominated from the start, slicing 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 number one 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 and keeping Hy-Boulais off balance from the baseline.
"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.
"I couldn't tell she had any shoulder problems,'' Hy-Boulais said. "Monica plays a different game than the rest of the pack. She stays right on top of the baseline and makes you alter a lot of shots."
Seles, who beat Martina Navratilova in her wins here in 1990 and 1992, could meet the other Martina in Sunday's final. Switzerland's Martina Hingis meets Wild in the quarter-finals.
Seles has only one seed, fourth-seeded Mary Joe Fernandez, in her half of the draw. Hingis has Schultz-McCarthy and Davenport in her section.Reuse content