It took a last-minute campaign blitz and a significant financial investment for Clinton to win the Kentucky Democratic primary by half a percentage point over Sanders - in a state she won by 35 percentage points over then senator Barack Obama in their 2008 primary clash, CNN reported.
Sanders easily won the Oregon primary, and declared at a raucous rally in California that despite pressure from the Clinton campaign to abandon his quest for the nomination, he would stay in the race "until the last ballot is cast".
Clinton did not appear in public on Tuesday night, but her campaign tweeted thanks to the people of Kentucky and said "we're always stronger united".
Clinton maintains an overall lead of 280 pledged delegates with only one significant night remaining in the contest - the California primary on June 7.
Clinton now stands at 2,294 votes (pledged: 1,773; superdelegates: 521), Sanders has 1,523 votes (pledged: 1,482; superdelegates: 41).
Sanders' campaign manager Jeff Weaver told CNN that the close race in Kentucky was a sign that a lot of Democrats "are having second thoughts" about Clinton.
"The media is ready to call this race over, but I think voters in the various states want to see this race go on," Weaver said.
Meanwhile, Donald Trump, the presumptive Republican Party's presidential nominee won the contest in Oregon which was moot since his remaining rivals have already dropped out of the race.
Still, the primary brought the billionaire to within fewer than 70 delegates of the 1,237 delegates he needs to formally claim the nomination.