Williams had initially targeted last month's Australian Open for a comeback, but abandoned that ambitious goal after declaring she was not 'where I personally want to be.
"She wouldn't have agreed to play if she wasn't ready", said Rinaldi, when asked about possibly seeing Serena in singles Sunday.
"It's definitely the start of a long process", Williams said. Physically I feel good. Every day, I get better and I get stronger.
Williams, 36, hasn't played an official match since winning the 2017 Australian Open.
Venus' win came in her 1,000th career match and her 22nd singles appearance in a tournament she first played in 1999, but she insisted milestones were far from her mind. The schedule originally called for Serena Williams to play alongside Lauren Davis. Sunday's schedule now has Venus Williams facing Hogenkamp and Vandeweghe taking on Rus, but both captains still have time to make substitutions.
Serena Williams makes a backhand return to her sister Venus during the women's singles final January 28, 2017 at the Australian Open.
Seven-time Grand Slam victor Venus Williams needed an hour and 19 minutes to brush aside her 124th-ranked Dutch opponent Arantxa Rus 6-1, 6-4.
The 37-year-old, who enjoyed a renaissance year in 2017 when she reached two Grand Slam finals, showed some signs of tiring towards the end of the second set even as her clear advantage in skill and raw power were on display.
She added: "Right now I'm focused on this weekend and after that I'll figure out what it might be". "It goes unsaid 25 (grand slams) is obviously something that I would love, but I'd hate to limit myself".
Dozens dive into cold Deschutes for Bend's Polar Plunge
Frigid temperatures Saturday didn't stop people from taking part in the Polar Plunge in Manhattan. This is just one of the fundraising efforts for the more than 16,000 Special Olympics athletes.
The anticipation of her return to tennis after the birth of her daughter, Alexis Olympia, was not lost on the local crowd. Williams who is married to Reddit co-founder Alexis Ohanian, said motherhood is "an wonderful journey".
"It's never a win until it's over", she said.
Venus Williams has been impressed with her younger sister's parenting skills.
"She's very professional and she has a lot of pride for playing for her country".
"We went onto the court and held the minute's silence which made us cry", Kvitova said afterwards. "I think that today's match hopefully will help me for tomorrow".
She mentioned there are "moments that have just been hard, getting back out there doing it every day".
But the 37-year-old insisted she had plenty of time to turn things around.
US captain Kathy Rinaldi said Sunday the Williams sisters would play doubles. Not only does it allow her to practice alongside a few different players, it also enables her coach and teammates to offer different perspectives on her progress.