Williams had not played a singles match since last year's US Open in New York when she revealed she had been diagnosed with Sjogren's syndrome, an auto-immune disease that causes fatigue and joint soreness.
But the 31-year-old American was back on the court on Wednesday, beating Japan's
"Just to be in this tournament is a huge win for me," Williams said after being granted a wildcard entry into the tournament.
"Just being here is a win, I told myself going into this, this is a win. Just to train and get back at this level."
Belgium's Clijsters, another former number one on the comeback trail, overcame a slow start before beating Australia's
Clijsters had not played since she lost to eventual champion Victoria Azarenka in the Australian Open semi-finals when she injured her ankle.
"It's always tough to get that first match rhythm under your belt," Clijsters said.
"One thing I can count on is I have the experience, though. It doesn't take me that long to get used to it again. But it has become tougher."
With none of the seeds scheduled to play until the second round on Thursday, there were also first round wins for