* Dollar sags as Republican senators seek tax bill changes
* Bitcoin surges towards $18,000 ahead of CME futures
* Kiwi up, NZ finmin says "comfortable" with currency's trend
* Graphic: World FX rates in 2017 http://tmsnrt.rs/2egbfVh
LONDON, Dec 15 (Reuters) - The dollar slipped to a nine-day low against the yen on Friday, after wrangling in the United States Congress over a bill to change the tax code dented confidence that the reforms would be pushed through in their current state.
Web-based cryptocurrency bitcoin stole the spotlight once again, surging 10 percent to a new record high just shy of $18,000 on the Bitstamp exchange ahead of the launch of bitcoin futures on the world's biggest derivatives exchange operator CME Group on Sunday.
The greenback had climbed to a one-month high of 113.750 yen on Tuesday. But it made a U-turn midweek after the Federal Reserve raised interest rates as expected but also expressed concern about low inflation.
The dollar lost further ground after two U.S. Republican senators on Thursday were reported to have sought changes to the proposed legislation to overhaul the U.S. tax code.
The U.S. currency slipped to 112.18 yen on Friday, down 0.2 percent on the day.
"The dollar is broadly weaker today, with the uncertainty over U.S. tax legislation the chief driver," said Societe Generale currency strategist Alvin Tan, in London.
"Also (the) yen is being helped by softer tone in equities."
The tax bill needs a simple majority to pass in the Senate, in which Republicans hold 52 of the 100 seats, with no Democrats are expected to support it.
Democrat Doug Jones won the contest for a U.S. senate seat on Tuesday in Alabama, a Republican stronghold, trimming the Republicans' already narrow Senate majority.
"In the end the effect will depend a lot on the details of the final tax bill," said Commerzbank currency strategist Esther Reichelt, in Frankfurt. "The more the tax bill gets watered down, the less pronounced the effect will be on the dollar."
The euro edged up 0.1 percent to $1.1792 after losing about 0.4 percent on Thursday.
The common currency flagged after the European Central Bank on Thursday raised growth and inflation forecasts for the euro area, but stuck with its pledge to provide stimulus for as long as needed.
The New Zealand dollar was the biggest mover among major currencies, up 0.6 percent at a two-month high of $0.7029 after the country's finance minister Grant Robertson said he was comfortable with the currency's general trend.
Bitcoin was up a massive 20 percent on the week, though trading has turned a little less volatile following the start of trading of Cboe Global Markets' bitcoin futures. (Reporting by Jemima Kelly; Additional reporting by Shinichi Saoshiro in Tokyo,; Editing by Catherine Evans)