NEW YORK (Reuters) -Frances Tiafoe held off fellow American Aleksandar Kovacevic to come away with a 6-4 6-3 4-6 7-5 win in the first round of the U.S. Open on Monday.

Tiafoe, seeded 20th, remained composed, even as the pressure ramped up, to advance as he looks to make good on his promise to win his home Grand Slam after his scintillating run to the semi-finals two years ago put him firmly on the map.

Kovacevic, a New York native making his U.S. Open debut, looked a little nervy serving in the 10th game of the opener and Tiafoe took full advantage, leaping high in the air to pound an overhead and grab the first set.

Tiafoe smoked a service return for a 4-2 lead in the second and took a 2-0 lead when Kovacevic’s one-handed backhand sailed long.

Kovacevic came to life in the third, hitting a sensational around-the-net forehand winner in the fourth game to peel off some of the support from the pro-Tiafoe crowd and breaking serve for a second time to force a fourth set.

But Tiafoe would not be denied, breaking for a 6-5 lead in the fourth and converting his fifth match point to seal the win.

Thunderstorms forced the closure of the roof on Louis Armstrong Stadium and Tiafoe said the change in conditions had a big impact.

“It got pretty tough there at the end,” Tiafoe said.

“When they closed the roof it was really muggy in here. I was really struggling breathing midway in the third and most of the fourth.

“At the end of the fourth I started feeling much better and I was trying to do everything I could to finish in four. But definitely tough, muggy conditions tonight.”

Tiafoe has struggled to find momentum and wins this season but has looked sharper of late, reaching the final of the Cincinnati Open last week to break back into the top 20.

“Cincinnati was such a great week, I’m just trying to gain steam from that,” he said.

Next up for Tiafoe is a second-round meeting with Alexander Shevchenko of Kazakhstan.

(Reporting by Rory Carroll in Los Angeles; Editing by Peter Rutherford)