Serena Williams says she is trying to "move on" from the meltdown that overshadowed her US Open final loss but remains perplexed at her coach's admission he illegally signalled to her.

Naomi Osaka has put her US Open tears down to shredded nerves and her natural awkwardness, rather than the tantrum thrown by Serena Williams that overshadowed a historic Japanese win.

World tennis chiefs need to clarify their position on in-match coaching to prevent another incident after Serena Williams' US Open meltdown, the head of the Australian Open said Monday.

Novak Djokovic clinched his third US Open title on Sunday with a 6-3, 7-6 (7/4), 6-3 triumph over Juan Martin del Potro, taking him level with Pete Sampras's mark of 14 Grand Slams.

After going from goofball to Grand Slam champion, Naomi Osaka can follow her historic US Open triumph by becoming Japan's first world number one, predicts Japanese tennis great Kimiko Date.

Serena Williams has been fined $17,000 by the US Tennis Association in the wake her outburst during a controversial US Open final loss to Japan's Naomi Osaka.

Novak Djokovic hailed Juan Martin del Potro as a "gentle giant" but he'll have no hesitation in attempting to cut his close friend down to size in Sunday's US Open final.

Naomi Osaka told US Open title opponent Serena Williams "I love you" just moments after she became the first Japanese woman to reach a Grand Slam final.

World number one and defending champion Rafael Nadal will put his weary body on the line against a merciless Juan Martin del Potro on Friday with a place in the US Open final at stake.

Serena Williams is taking nothing for granted as she heads into the US Open semi-finals on Thursday with a record-equalling 24th Grand Slam title on the horizon.

Five-time champion Roger Federer crashed out of the US Open on Monday, stunned by 55th-ranked Australian John Millman who booked a quarter-final clash with Serbian star Novak Djokovic.

Maria Sharapova was knocked out of the US Open in the fourth round on Monday, losing 6-4, 6-3 to Spanish birthday girl Carla Suarez Navarro who inflicted the Russian's first ever night-time loss at the tournament.