Having battled to victory in her opening match at the WTA Finals in Singapore, the five-time grand slam champion is now in pole position to qualify from the Red Group after beating top seed Simona Halep on Tuesday.
The world No. 4 triumphed 6-4 6-4, winning seven out of eight games from 4-4 in the first set before holding off a late rally by the Romanian.
"It definitely got really tough," said Sharapova, who completed just her second match since losing to Serena Williams in July's Wimbledon semifinals.
"I guess those are the moments you have to get through when you haven't played in a long time, and against the No. 2 player in the world these matches don't come easy. It definitely showed until the end."
Sharapova became just the second player, after Williams, to reach 20 match victories at the season-ending championships.
She now has a 6-0 record over last year's runner-up Halep, who had been in stunning form on Sunday when she opened her campaign with a straight-sets victory over U.S. Open champion Flavia Pennetta.
"She did amazing shots in the most important moments in the match," said Halep, who has won three titles and lost two finals this year.
"Even if she didn't play for a long time, she knows how to play. She's a champion. She has experience. And she's always playing really well against me.
"But I will work harder. Now I'm more motivated that I have to learn things to beat her."
With her long-time nemesis Williams having taken a break until next year following the U.S. Open semifinal defeat that ended her hopes of a calendar grand slam, Sharapova has a great chance of winning the WTA title for the first time since triumphing as a teenager in 2004. That year was the last time she has beaten Serena, succumbing in their 17 subsequent encounters.
She lost to the American in January's Australian Open final, and won titles in Rome and Brisbane, but missed the U.S. Open with a leg problem and suffered an arm injury during her comeback match in Wuhan at the end of last month.
The 28-year-old, also the WTA Championship runner-up in 2007 and 2012, will next face Pennetta -- who is making her debut as a singles player.
The Italian, who will retire at the end of this season, kept her semifinal hopes alive by beating Agnieszka Radwanska 7-6 (7-5) 6-4 earlier Tuesday.
The 33-year-old battled back from 5-3 down in the opening set against Poland's former world No. 2, who was on court for almost three hours against Sharapova on Sunday.
"She plays unbelievable -- she can let you run for hours all over the court, so you need to be really aggressive with her," Pennetta said. "After this win, I still have a little bit of chance. It's good to be in this position, to have a chance to play another match and get into the semifinals."
On Wednesday. Petra Kvitova and Lucie Safarova will both be seeking their first wins in the White Group, in an all-Czech left-handers clash.
Spanish second seed Garbine Muguruza and Germany's Angelique Kerber will be vying to claim a second victory following their Monday successes in the eight-woman event.