By Steve Keating

NEW YORK (Reuters) - A new roof was all the rage when the U.S. Open began two weeks ago but it was the crowning of a new world number one and two first-time champions that left their mark on a tournament that came to an emotional close on Sunday with a tribute to victims of the Sept. 11, 2001 attacks.

At a tournament where trainers had nearly as much court time as weary players, Germany's Angelique Kerber and Switzerland's Stan Wawrinka were the last two standing in their respective draws as they each captured their first U.S. Open titles.

Wawrinka played with energy and desire as he tamed defending champion and world number one Novak Djokovic 6-7(1) 6-4 7-5 6-3 in a victory that was a testament to his will as much as skill and earned him his third grand slam title.

Djokovic, contesting his sixth U.S. Open final in seven years, benefited from one of the easiest passages to a grand slam final as he was handed a walkover and two retirements but it was the Serb who was ultimately worn down by Wawrinka.

The nine hours Djokovic spent on court to reach the final were half the 18 hours logged by the battle-hardened 31-year-old Wawrinka, who added a U.S. Open title to a grand slam collection that also includes French Open and Australian Open trophies.

Kerber, another player with a deep well of stamina, tapped into those reserves on a sweltering Arthur Ashe Stadium court to defuse big-hitting Czech Karolina Pliskova 6-3 4-6 6-4 to capture her second grand slam title.

And while Kerber may be a new U.S. champion, the 28-year-old German is hardly a new face in the world of tennis.

She has become one of the game's most recognizable figures in a season that has included an Australian Open triumph, a run to the Wimbledon final, an Olympic silver medal in singles and ending Serena Williams's 186-week reign as world number one.

While the buzz at the start of the U.S. Open was about the $150 million retractable roof over Arthur Ashe Stadium it was the Flushing Meadows wall that Williams ran into for a second consecutive straight year that drew the most attention.

A year ago, on the cusp of a rare calendar-year grand slam, Williams was felled by 300-to-1 longshot Roberta Vinci.

This year her quest for a record-breaking seventh U.S. Open title was again undone in the semis, this time by Pliskova, who arrived at Flushing Meadows as a grand slam underachiever who had never been beyond the third round of any major.

Flushing Meadows did not exactly witness a passing of the torch this year but the winds of change did blow through the Billie Jean Tennis Center.

Djokovic was the only member of the so-called 'Big Four' in men's tennis to reach the semi-finals while the women's draw produced a first-time champion for the second straight year following Flavia Pennetta's magical run in 2015.

Since 2005 when Rafa Nadal won his first French Open title, the Spaniard, Djokovic, Roger Federer and Andy Murray have accounted for 42 of 46 grand slam titles.

There were fresh faces like 18-year-old Croatian Ana Konjuh, who upset fourth seed Agnieszka Radwanska, but it was another false dawn for U.S. tennis.

At the start of play, 39 Americans were in the main draw, one fewer than the record set by local players in 2005, but by the second week all were gone except the aging Williams sisters, Madison Keys and Jack Sock.

The American flag, however, did wave proudly over Arthur Ashe Stadium on Sunday as the U.S. Open staged a remembrance for the victims of the Sept. 11 attacks on the United States.

With 9/11/01 stenciled onto the Arthur Ashe Stadium court, American Bethanie Mattek-Sands paid her own tribute by wearing Stars and Stripes knee-high socks and wrist bands.

"It is an emotional day," said Mattek-Sands, who won gold at the Rio Olympics in mixed doubles with Sock. "I was going to retire my American flag socks after the gold medal match.

"I said, You know what? For the final of the U.S. Open, 9/11, have to bring them out.

"It was really special for me to win it here today in New York."

(Editing by Frank Pingue)