By Steve Keating

NEW YORK (Reuters) - Croatia's Mate Pavic and Germany's Laura Siegemund claimed the first silverware of the U.S. Open on Friday, landing the mixed doubles title with a 6-4 6-4 upset win over the seventh-seeded U.S. partnership of Rajeev Ram and Coco Vandeweghe.

It was the first grand slam title for the unseeded Pavic and Siegemund, who swept through all 10 sets and three tie-breaks to hoist the trophy.

Siegemund, seeded 26th in the women's singles at the U.S. Open, lost to Venus Williams in the third round.

The U.S. Open also marked the first time that Vandeweghe and Ram, who won a silver medal with Venus Williams at the Rio Olympics, had teamed up.

It was further grand slam disappointment for Vandeweghe, who lost in the final of this year's Australian Open mixed doubles with Horia Tecau.

(Editing by Mark Lamport-Stokes)