Less than a day after sending out a rare personal tweet to congratulate the most decorated Olympic athlete ever, Obama called Michael Phelps on the pool deck to hail his performance.
Phelps, still reeling from the previous day's tweet, described the call as "pretty cool".
"I answered the phone and the voice said 'Michael?' 'Yes.' 'Please hold for the President of the United States," he told reporters on Thursday, after qualifying for the 100 metres butterfly semi-final.
"I was like, 'wow'."
The president, he said, told Phelps the country was supporting him and proud of his performance.
"He finished by saying 'make sure you tell your mum I said hi'."