Sixth-seeded Spaniard Muguruza needed only 56 minutes to capture her second title of the year and her first in a US event, taking the $522,450 top prize in the last major US Open tuneup.
It marked the third loss of the year for Halep when she was playing for the top ranking, also falling in the French Open final against Jelena Ostapenko and the Wimbledon quarterfinal against Johanna Konta, each time after taking the first set.
Halep apologized to spectators after the match, thanking them for support “even if I played so bad and Muguruza played so well.”
Czech Karolina Pliskova, a semi-final loser to Muguruza at the last major US Open tuneup, will remain world number one by a five-point margin over Halep, who would have become the 24th different number one.
Serena Williams had been number one before her pregnancy break earlier this year. Germany’s Angelique Kerber and Pliskova have held the spot since.
Muguruza, who will jump to third in Monday’s rankings, becomes a favorite to capture a third Grand Slam crown in Flushing Meadows after taking Wimbledon and last year’s French Open.
Muguruza, 25, broke Halep’s first service game for a 2-0 lead and Halep double faulted away another break in the sixth game before Muguruza held to claim the first set in 23 minutes. Halep had 15 unforced errors in the set to six for the Spaniard.
Halep, 23, smacked a forehand long to surrender a break to open the second set, swatted a backhand long for another break that gave Muguruza a 3-0 edge.
In the 12-minute fourth game, Halep netted a forehand and sent a backhand wide to squander her only break chances. Muguruza broke again in the fifth game and held serve to claim the crown. (Jim Slater/ AFP)