Caroline Wozniacki has congratulated Serena Williams on her 18th Grand Slam title, saying, "Unfortunately I lost to a better player".
The Danish tennis star wrote on Twitter.
Serena Williams struggled to hold back the tears as she explained the "wonderful feeling" of winning a sixth US Open title 15 years after her first.
As Williams collected a cheque for $4 million (£2.5 million), the biggest in tennis history, she said: "It was a wonderful finish, I couldn't have thought of any better way.
"Congratulations to Caroline [Wozniacki], she's known what a struggle it's been for me, we text almost every day.
"I know you're going to win [a Grand Slam) very soon, maybe even in Australia. I've got to go home and get fit again so I can get ready for you."
Serena Williams has won her third consecutive US Open women's championship after defeating Denmark's Caroline Wozniacki 6-3, 6-3.
The Grand Slam title was the 18th of Williams' career but the American's first of this year.
Serena Williams was forced to pull out of her doubles match with sister Venus at Wimbledon after she became unwell on court.
In scenes which were often uncomfortable to watch, the American appeared dizzy on court and was unable to hit or bounce the ball.
After she served four consecutive double faults, umpire Kader Nouni came down to speak to the sisters as the worrying situation continued.
The sisters eventually forfeited the match against German Kristina Barrois and Swiss Stefanie Voegele after going 3-0 down in the first set.
The commentator said Williams, who has won five Wimbledon singles titles, was a "shadow of herself" and appeared to be "sleepwalking" through the match.
Defending Wimbledon champion Serena Williams has been knocked out of Wimbledon by German Sabine Lisicki.
Lisicki won 6-2 1-6 6-4 to set up a quarter final clash with Kaia Kanepi who defeated British hopeful Laura Robson earlier today.
Williams's exit follows a host of shock exits last week with top seeds Roger Federer, Rafael Nadal and Maria Sharapova all going out.
Serena Williams launched a charm offensive as she sought to broker a truce with Maria Sharapova and calm the storm surrounding comments she made about a rape victim.
The focus of the pre-Wimbledon press conference was about the interview she gave to Rolling Stone magazine that touched on a high-profile teenage rape case in Ohio and brought her into conflict with her Russian rival Sharapova.
The defending champion said she personally apologised to Sharapova and told her she was "very sorry for this whole situation":
Tennis champion Serena Williams has apologised over comments she made in a forthcoming interview in which she appeared to assign blame to the 16-year-old victim in an Ohio rape case, for being drunk.
The comments were reported in an interview with Rolling Stone magazine and stirred up US media attention
"What happened in Steubenville was a real shock for me. I was deeply saddened," the women's world number one said in a statement on her website.
"What was written - what I supposedly said - is insensitive and hurtful. I by no means would say or insinuate that she was at all to blame.
"I have fought all of my career for women's equality, women's equal rights, respect in their fields - anything I could do to support women I have done."
The small Ohio town of Steubenville was rocked when the young victim accused two high school American football players of raping her while she was drunk at a party and testified that she remembered little of what happened in the early hours of 12th August, 2012, when she says she was assaulted.
Serena Williams captured another gold for USA at the Olympics with a straight-sets thrashing of Maria Sharapova in the women's singles final.
Williams won 6-0, 6-1 on Wimbledon's Centre Court to leave the Russian settling for silver.
It was an historic performance from the American, who becomes the first woman to win all four grand slams and an Olympic gold in both singles and doubles.