US Vice-President Joe Biden signed the condolence book for Mr Lee Kuan Yew on Thursday morning in Washington, the highest-ranking US leader yet to visit the Singapore embassy to pay respects.
In a page-long message, Mr Biden said that few world leaders possessed Mr Lee's insight and wisdom on world affairs.
"One of my fondest and lasting memories was meeting with the prime minister in Singapore in 2014. We discussed the relative position and prospect of India, China, Russia and the United States - the breadth and depth of his understanding impressed me. My only regret was that I didn't have a full week just to ask him questions regarding world affairs," he wrote.
A series of leaders have signed the book since it was opened on Monday. They include former secretary of state Colin Powell, Deputy Secretary of State Tony Blinken, and dozens of representatives from the diplomatic core in Washington.
The US is sending a high-level delegation to Singapore for the funeral led by former president Bill Clinton.
Mr Biden has been known to hold Mr Lee in high regard and in recent years, has often spoken about his meetings with the Singaporean leader in public speeches.
He added on Thursday: "He spoke warmly of the friendship between our nations. He admired the virtues of America and the importance of the enduring US role in Asia Pacific. He made and had a profound impression on me. I'll never forget, may he rest in peace."