Umpire Ramos gave Williams a code violation after her coach gestured in her direction early in the second set of Naomi Osaka's 6-2, 6-4 victory over the American.
Serbia's Novak Djokovic jumped three spots to third after he captured his third US Open title and 14th overall grand slam.
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.
Naomi Osaka raced to a 3-0 lead in the first set and then 4-0 in the second, never giving the shaky Tsurenko a chance to get into her first major quarter-final.
The defending champion and No. 1 seed at Flushing Meadows recovered from his disastrous start and other stumbles along the way to beat No. 9 Dominic Thiem.
Williams put aside some early shakiness and an early deficit, turning things around with an eight-game run en route to a 6-4, 6-3 victory over No. 8 seed Karolina Pliskova for a spot in the semifinals.
Maria Sharapova won the US Open as a teenager. More than a decade later, she can't seem to make it past the fourth round.
Roger Federer suffered a shocking 3-6, 7-5, 7-6 (7), 7-6 (3) defeat to 55th-ranked John Millman of Australia in the fourth round of the 2018 US Open.
Williams reached the quarter-finals at Flushing Meadows for a 10th consecutive appearance by picking her level up after a lull and using 18 aces to beat Kaia Kanepi of Estonia 6-0, 4-6, 6-3.
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