Fired-up fifth seed Simona Halep reached the third round of the US Open on Thursday with a straight-sets victory over Lucie Safarova in a battle of former French Open finalists under the new roof of Arthur Ashe stadium.

In the first match to be played from start to finish under the closed roof, Romania's Halep triumphed 6-3, 6-4.

Earlier on Wednesday, Rafa Nadal boasted after his first-round U.S. Open match that he had a video recording of being the first player to hit a ball in practice under the new roof at Arthur Ashe Stadium.

The fourth-seeded Spaniard achieved the double as he hit the first ball in competition under the $150 million roof after rain briefly interrupted his second-round match against Italian Andreas Seppi.

The crowd looked skyward at the spectacle of the high-tech closure and roared in approval when the two sides met above the court in five minutes, 35 seconds.

Play resumed seven minutes, 22 seconds after the stoppage with Nadal serving at 3-3 in the second set before he completed a 6-0 7-5 6-1 victory over Seppi that lifted him to a third-round match against Russian Andrey Kuznetsov.

The Spaniard was aglow after his victory and being part of the first match played under the Ashe Stadium roof.

"I was very happy to be the first player to play in the competition with this fantastic roof closing," he said. "Amazing, unbelievable."

Nadal's mood was in stark contrast to women's French Open champion Garbine Muguruza who had preceded her compatriot onto the Arthur Ashe Stadium court and was upset by 48th-ranked Anastasija Sevastova of Latvia 7-5 6-4 in the second round.

Muguruza and Sevastova played with the roof open, but both complained about the noise from the boisterous fans, which appeared to be exacerbated by the roof support structure trapping the sound.

Meanwhile, Milos Raonic became the first major casualty of the U.S. Open when American qualifier Ryan Harrison shocked the Canadian fifth seed 6-7(4) 7-5 7-5 6-1 on Wednesday to reach the third round of the year's final grand slam.