Keys, who reached the Australian Open semi-finals and Wimbledon quarter-finals last year, reined in her sometimes over exuberant groundstrokes to claim a 4-6 7-6(5) 6-2 win over the 60th-ranked Riske in a grueling two hour, 25 minute slugfest.
With the night program delayed by more than an hour following a ceremony to mark the opening of the tournament, the final match ended at 1:48 a.m., making it the latest women's match ever at the U.S. Open.
"I just looked up and it's almost 2 (a.m.)," the 21-year-old Keys laughed in a post-match interview. "Who wants to go party?"
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After years of hand-wringing over the state of U.S. tennis other than mighty women's world number one Serena Williams, talent from the junior ranks is beginning to emerge.
At the start of play, 39 Americans were in the main draw, one fewer than the record for local players set in 2005, and by the end of the day, nine of them had advanced but 14 had already been eliminated in first round action.
Next up for Keys, who twice received treatment on her right shoulder, is another American, 16-year-old Kayla Day, the U.S. national junior champion and youngest player in the draw.
UPSETS AVOIDED
On the men's side, two established American players, John Isner and Jack Sock, avoided upsets, fighting off challenges from highly touted 18-year-old compatriots.
Big-serving Isner, the top-ranked American in the draw at number 21, roared back from two sets down to beat Frances Tiafoe 3-6 4-6 7-6(5) 6-2 7-6(3).
In another all-American battle, 26th-ranked Sock, who won mixed doubles gold and doubles bronze at the Rio Olympics, held off Taylor Fritz 7-6(3) 7-5 3-6 1-6 6-4.
The winners were full of praise for their younger opponents.
"I was struggling to match his intensity," Isner, 31, said about the Grandstand court contest. "In the early going, he was all over me and was the better player hands down.
"The atmosphere was amazing. Standing room only in that fifth set. The crowd was going nuts."
Tiafoe was serving for the match at 5-3 in the fifth set, but Isner broke to bring it back on serve and won the deciding tiebreaker 7-3.
"I can't say enough about Frances," Isner said. "He has a fantastic future."
Sock was equally as magnanimous about Fritz after his win.
"He's an incredible young player, has an incredible future ahead of him," the 23-year-old Sock said.
(Editing by John O'Brien)
