She will face Caroline Wozniacki of Denmark on Tuesday night. The 16-year-old Wozniacki took three sets to beat Stephanie Foretz of France 2-6, 7-6 (5), 6-3 earlier Monday.
"I definitely was a little rusty, and the first set went so quick, it was fairly easy," Williams said. "In the second set, I started to think and I made a few errors. It seemed like I couldn't stop the errors. It was like a windfall of errors."
Williams was playing in her first tournament since October and had played only two matches since July because of the sprained left wrist. The 26-year-old right-hander, ranked 53rd in the WTA rankings, had withdrawn from two tournaments earlier this year, including the Australian Open.
She declared the wrist ready last week and said she was ready to return to the court. But Williams dealt with some long rallies and poor hitting during the 1 hour, 45 minute match. Those factors, along with an erratic serve, kept Morigami in the match.
"I'm really confident at this point," Williams said after putting the wrist through a competitive test. "I didn't really know what to expect in the first match."
Williams' serve, which contributed to her getting 60 percent of first serves in play, got erratic in the second set. Morigami broke Williams three times in the set, including the fourth game when Williams recorded five of her eight double faults.
Stamina may not have been as much of a factor as the mental part of her game. She said at times the focus wasn't where it should have been, particularly after the easy time in the opening set.
"I had a lot of things going on in my head, and it's just a matter of getting back to the game flowing," Williams said. "Sometimes I was feeling that, and sometimes I wasn't."
Morigami said when it was time, Williams showed she was the better player.
"She still was hitting a pretty good serve when she needed to, and that was the difference, I think," Morigami said.
Unseeded American Bethanie Mattek defeated two-time champion Lisa Raymond 6-2, 7-5 in the first round. The 33-year-old Raymond was a back-to-back winner of the CSC in 2002-03, but struggled against Mattek.
No. 5 seed Nicole Pratt beat Lilia Osterloh 6-3, 6-0 to advance to the round of 16.
On the men's side, fifth-seed Xavier Malisse of Belgium retired in the second set of his match with Teimuraz Gabashvili of Russia with a right wrist injury. Malisse, who was tied 2-2 in the set after winning the opening set 6-4, will have an MRI on Tuesday.
In a match late Monday night on the men's side of the tournament, Russian Evgeny Korolev defeated eighth-seeded American Robby Ginepri 7-6 (4), 4-6, 6-3.
Also on Monday, sixth-seeded Austrian Jurgen Melzer rallied to beat American qualifier Jesse Witten 1-6, 6-3, 7-6 (3) in the first round of the Regions Morgan Keegan Championships.
The 24-year-old Witten, a regular on the Challenger circuit, needed just 21 minutes to take the first set from his Austrian counterpart. Melzer regrouped in the second and then won a third-set tiebreaker despite some lingering pain from his injured left wrist.
"Losing this match would have been a tough one for me because I think I'm the better player," said Melzer, ranked No. 31 in the world.
The 203rd ranked Witten displayed a powerful forehand while breaking Melzer's serve on three occasions.