The Romanian world No. 2, playing her first event since winning in Dubai in February, won 6-2, 7-5.
However, she found her groove against Mertens.
Victory in one hour 33 minutes over Belgian Mertens earned her a 21st career Women’s Tennis Association trophy — the fifth-highest tally of active players behind only Serena Williams, Venus Williams, Kim Clijsters and Petra Kvitova.
Mertens blazed into a 2-0 lead with a flurry of winners, but Halep responded by reeling off seven successive games to take control.
Halep did not have things all her own way though and was broken back early in the second set as Mertens again went on the attack. A wobbly service game allowed Halep to move a break ahead again in the fifth game, alhough Mertens retaliated again to level the second set at 4-4.
Halep broke to lead 6-5 though and finished it off with a heavy forehand.