Halep lost the first set 6-1 to Tunisia's Ons Jabeur before retiring with a back injury that she also had problems with at last week's Wuhan Open.
Roger Federer earlier put Team Europe ahead by saving three match points for a 6-7, 7-6, (10-7) victory over John Isner.
Pliskova broke Osaka for a second time to go up 5-4 in the second set and then hammered her sixth ace to secure her 11th career title.
India are currently on top with four golds, five silvers and a bronze medal in the competition.
Federer and Djokovic took the first set 7-6 in a tiebreaker, but Sock and Anderson won the second set 6-3 and then captured the deciding tiebreaker 10-6.
Osaka hammered nine aces over the course of the victory and dropped just eight points in eight service games over the course of the match.
They lost 6-7(5), 2-6, 6-7(4) to relatively inexperienced pair of Nikola Milojevic and Danilo Petrovic after two hours and 22 minutes on indoor courts.
Djere, ranked 86, had lost both his earlier Davis Cup matches before clashing with Ramkumar, ranked 135, at Kraljevo Sports Venue.
The Grand Slam Board announced on Thursday that it has given up its earlier-stated intention to revert to 16 seeds in 2019.
Carlos Ramos, the chair umpire who penalized Williams is also handling their best-of-five semifinal series against Croatia this weekend.
Recently-crowned US Open champion Novak Djokovic's decision to opt out of the playoffs along with world number 33 Filip Krajinovic has raised hopes of an equal contest between the two teams.
The Australian cartoonist behind the controversial depiction of Serena Williams jumping up and down on a broken racket has defended himself against claims of racism.
Serbia's Novak Djokovic jumped three spots to third after he captured his third US Open title and 14th overall grand slam.
Hearing those delirious Argentine fans sing "Ole! Ole! DelPo!" again between points, gave him something else rewarding to take home from Flushing Meadows.
US Open Men's champion Novak Djokovic said he thought chair umpire Ramos should not have pushed Williams so hard.
A year after missing the U.S. Open because of an injured right elbow that would require surgery, Djokovic showed that he is unquestionably back at his best and back at the top of tennis.
Williams had lost the final 6-2, 6-4 to Japan's Naomi Osaka but the game made the headlines for her allegation that Ramos treated her differently than he would a male player.
For the first time in the United States Open, the trophy ceremony was drowned out by whistles and boos in support of the loser and against the champion.
Williams was cited by official Carlos Ramos for three code violations during her 6-2, 6-4 loss to Naomi Osaka in US Open finals.
Osaka grew up rooting for Serena Williams, even did a report on her way back in third grade. Her dream was to play Serena at the U.S. Open.
The pair won its second straight Grand Slam doubles title together, beating Lukasz Kubot and Marcelo Melo 6-3, 6-1 on Friday at the U.S. Open.
On Sunday, No. 3 del Potro will face No. 6 Novak Djokovic, who advanced with an emphatic 6-3, 6-4, 6-2 victory over No. 21 Kei Nishikori in the second semifinal.
Del Potro was leading 7-6 (3), 6-2 after two hours of play when Nadal shook his head and said he had to stop.
On Saturday, Williams will face No. 20 seed Osaka, a 20-year-old who is the first Japanese woman to reach a Grand Slam final.
All four players have reached the U.S. Open final while only Nishikori hasn't won it to make this the first Grand Slam final four since the 2012 Australian Open in which all four players were past finalists.
Djokovic moved a step closer to a third championship at Flushing Meadows and 14th Grand Slam title overall by eliminating John Millman, who conquered Roger Federer in quarterfinals of US Open 2018.
Both players won, Osaka decisively and Nishikori narrowly to give Japan a men's and women's semifinalist at the same Grand Slam tournament for the first time.
The 14th-seeded Keys reached her third semifinal in the past five majors by using her big-strike game built on serves and forehands to overpower no. 30 Carla Suarez Navarro.
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