The Polish pro secured a 7-6(5) 6-1 victory in Singapore to prevent Halep's progress into the knockout rounds.
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It was a troubling start for Radwanska when she lost her opening service game, but after battling to save two break points in the fifth game, she went a break up herself to draw the set level on 3-3.
Halep, who is the number one seed, came close to taking a 5-4 lead when she created three break points, but she failed to convert any of them.
Radwanska had the chance of set point on Halep's serve, but the Romanian managed to hold and a tie-break was eventually required to separate the pair.
After taking the breaker 7-5, Radwanska stormed to a 4-1 lead by breaking her opponent's serve twice, leaving Halep with a mountain to climb.
In the end, it was a comfortable set for Radwanska, who lost her two previous matches, as she wrapped up the contest.