Halep, the top seed and world number two, thrashed US Open champion Flavia Pennetta 6-0 6-3, before Sharapova saw off Agnieszka Radwanska 4-6 6-3 6-4.
The eight-player season finale begins with two round-robin groups, the top two from each qualifying for the semis.
World number one Serena Williams is absent as she recovers from injury.