"It's basically following yields in the U.S., which are grinding lower as the market gets a little bit nervous ahead of Thursday."
With the Bank of Japan keeping 10-year Japanese bond yields pinned to zero, the dollar-yen exchange rate has been closely correlated for the past year with any shifts in U.S. 10-year Treasuries.
The dollar index, which tracks the greenback against six major rivals, was down 0.25 percent at 96.56.
Comey, fired by President Donald Trump in May, will be grilled by the Senate Intelligence Committee on whether Trump tried to get him to back off an investigation of alleged ties between his 2016 campaign and Russia.
The threats to Trump and some of his senior advisers from the affair have added to market doubts over the administration's ability to deliver a promised boost to growth and weighed on the dollar.
The euro was up 0.20 percent against the dollar to $1.1275. The common currency has gained in recent weeks on various factors, including an ebb in French political concerns and upbeat euro zone data.
However, complacency that the ECB will be less dovish has left the euro vulnerable to disappointment if the central bank does not satisfy those expectations, analysts said.
Sterling was down 0.09 percent against the dollar to $1.2889. With two days to go until Britain holds a national election, opinion polls have shown that Prime Minister Theresa May's lead over the opposition Labor party has ebbed over the last three weeks, with some putting her
majority in doubt.
"This has put sterling under pressure as investors are looking to cover their risk," said Naeem Aslam, chief market analyst for Think Markets in London.