The Romanian had won the trophy in Dubai prior to the shutdown, and has enjoyed a fine week on clay - showing her fighting spirit to win in three sets twice, and overcoming a break deficit against Irina-Camelia Begu in the semi-final.
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This time round she logged a straight-sets victory against the Belgian, who had won five of her six previous WTA finals - and had beaten Halep in the Doha final last year.
Mertens had broken at the start of the match, but could not hold on to that lead in the face of the aggression from the reigning Wimbledon champion.
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Halep, 28, now has 21 WTA titles - the fifth-highest tally of active players behind only Serena Williams, Venus Williams, Kim Clijsters and Petra Kvitova.
She told the media afterwards that she had not yet decided whether or not to play the US Open in light of the coronavirus pandemic.
"We felt really safe here and all the rules were respected," she told reporters. "It's a bit stressful but the fact that everyone was good these 10 days means everything was done well."
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