The couple began dating at the beginning of last year (12), and the U.S. athlete's relationship with Rossee heated up just before he jetted off to the London Summer Olympic Games in July (12), when the pretty blonde was spotted cheering him on.
But both stars are now set for a fresh start in the New Year after deciding to part ways, according to TMZ.com.
Phelps appeared to confirm the news in a cryptic post on Twitter.com, which reads, "Things happen for a reason."
The 27 year old has previously been linked to American reality star Brittny Gastineau and model Nicole Johnson.
The Keeping Up With The Kardashians beauty is set to wed New Jersey Nets basketball player Kris Humphries next month (Aug11) and she celebrated her upcoming nuptials with a girls' night out at celebrity hotspot Tao.
Kardashian was joined by her mum Kris Jenner, sisters Khloe and Kourtney, TV personality LaLa Vasquez and socialite Brittny Gastineau.
But she couldn't stay away from her fiance, who was enjoying his own bachelor bash at nearby club Lavo, and gatecrashed Humphries' event for a few minutes towards the end of the night.
The couple announced its engagement in May (11) after a whirlwind six-month romance.
The socialite is engaged to marry her basketball player boyfriend Kris Humphries later this summer (11) and her sisters and close friends are planning to celebrate the impending nuptials with a bridal shower in Los Angeles on Friday (22Jul11).
They will then fly to Las Vegas for a wild night out on Saturday (23Jul11).
Kim's sister Kourtney opened up about their plans during an appearance on Lopez Tonight, telling the host, 'We are going to do whatever we can to embarrass Kim, just to bring her such embarrassment. That's our job... We're going to do it all. We have so many tricks up our sleeves that she won't even know what is happening."
Family friend, socialite Brittny Gastineau, even compares the upcoming party to Vegas-based movie The Hangover. She says, "Like The Hangover. All you need is that one night. Can you imagine if it were like The Hangover? If we all woke up and we were like, 'Where are we?'"
The Keeping Up With The Kardashians star teamed up with toy supplier HobbyTron to hand out games to youngsters at the Children's Hospital of Los Angeles.
Kardashian was also joined by reality star Brittny Gastineau as they met patients and posed for pictures with the kids.
In a series of posts on her Twitter.com page, Kardashian writes, "Heading to the LA Children's Hospital to give out toys! Thank u Hobby Tron for the amazing donation! 500 toys! Wow. Today was a heart warming day!"
The socialite and mother of Brittny Gastineau, her co-star on their family show Gastineau Girls, revealed all in a series of tweets to fans online, blaming her own clumsiness for the incident.
Taking to her Twitter.com page, she writes, "Clumsey (sic) me tripped in the dark and fell up the steps. Put my hands out to break the fall - got to meet all the cute ER docs.
"I can now definitely say that broken fingers hurt more than a broken heart."
Lisa Gastineau hit the headlines earlier this year (10) after Brittny broke the news of her mum's secret skin cancer battle. The 50 year old underwent surgery to remove cancerous cells from her face last October (09) and went under the knife again in March (10) to rebuild her nose following the medical treatment.
The socialite's daughter and Gastineau Girls co-star Brittny broke the news of her mother's condition at the launch of her new jewellery line in Los Angeles on Thursday (04Mar10).
And Brittny has revealed the ops to remove cancerous cells in October (09) left her glamorous mum feeling "ugly", so she put her beauty in the hands of Hollywood surgeon Dr. Frank Ryan, who cosmetically overhauled fellow reality TV star Heidi Montag earlier this year (10).
Gastinaeu tells New York gossip column Page Six, "She underwent two surgeries in October... When she was stitched up, it pulled her nose to the left - so she felt deformed and ugly. But a few days ago, Dr. Frank Ryan fixed her nose, and she is recovering well.
"My mom is my best friend. I don't know what I'd do if anything happened to her. I want to spread the message that people should check their skin carefully and stay out of the sun. We had a sun bed at home, and we'd use it all the time, and look what happened. It's really scary."
The socialite's daughter and Gastineau Girls co-star Brittny broke the news at the launch of her new jewellery line in Los Angeles on Thursday (04Mar10).
She told HollyScoop.com, "My mum can't be here right now. She has skin cancer so just had surgery."
And Brittny insists the family has been left stunned by the news: "My mum was diagnosed with Melanoma in 2006. Since then she has always had to check her skin. She had her basil cell removed from her nose in October so her surgery is reconstructive. She is fine, everyone is good. She is so tough, she really is the toughest woman ever. She is so inspirational to me."
The Danish star had a fling with the New York Jets quarterback in 1988 while he was still married to wife Lisa. She went on to have his child, Killian, in 1989 before fleeing the U.S. to live in Italy.
And despite knowing the child was his, the sportsman kept his estranged son a secret from his daughter Brittny for 20 years - until the reality TV socialite's mum bumped into Nielsen at Ivana Trump's wedding last year (09).
Brittny tells New York gossip column PageSix, "Brigitte literally said to my mom at Ivana's wedding: 'I have a son who wants to meet his sister.' Meeting my half-brother for the first time was really very weird and very emotional. The brother I never knew I had - it's a lot to take in... All my life, I've always said I wanted a brother. I definitely want us to be close."
And Brittny is upset that she has missed out on so much time with her brother: "I feel sad that I've had a brother for 20 years that I never knew growing up. He's very tall and blond, and looks identical to Brigitte. He doesn't speak much English because he grew up in Italy. He's really sweet."
Reality TV star KIM KARDASHIAN hosted a baby shower for her pregnant sister KOURTNEY on Saturday (14Nov09). More than 80 guests, including BRITTNY GASTINEAU, attended the party held at the family home in Hidden Hills, California.
The heiress appears in the outrageous new film, in which comic Sacha Baron Cohen plays a gay Austrian fashionista, as part of the funnyman's ploy to make a mockery of Hollywood.
In one shocking scene, Gastineau is shown an alleged sonogram of Jamie Lynn Spears' unborn foetus. When asked if she thinks the baby looks deformed, she agrees and when quizzed over whether Spears should terminate the tot, she shouts out, "Abort it!.
But the 26 year old is adamant she wasn't taken in by Cohen's camp character - claiming she was "spoofing herself" in the comedy.
She tells Usmagazine.com, "I was joking around. I was spoofing myself.
Gastineau adds that she was at first confused at being approached by a supposed European TV presenter, but went along with it because her show the Gastineau Girls is shown all over the world.
She explains, "I was confused; I thought maybe it was still playing over there. So I was like, 'Okay'. When I got there, I saw him, and I obviously knew it was him. I was like, 'Oh, this is funny. This is a joke.' I just went along with it.
"Everyone knew I wasn't serious when I said, 'Abort a baby!' I was just playing along. Anyone who is going to see Bruno knows it's a comedy, and I was totally joking around! I was not serious.".