The American world number one shot seven birdies and an eagle as he moved to 16 under and overtook Patrick Reed.
Reed (69) is joined on 12 under by fellow American Kevin Kisner (65) and Argentina's Fabian Gomez (66).
Irishman Padraig Harrington (68) is eight under and Australia's US PGA champion Jason Day is three under (73), alongside Scotland's Russell Knox (72).
Northern Ireland's Graeme McDowell is one under after a 70 at the Plantation Course in Kapalua.
Masters and US Open champion Spieth, who is bogey-free after his first two rounds, said: "It is nice to have no blemishes on the card. I want to prepare and start the year with a bang, but I'm only through 36 holes. There's a long way to go."
Rickie Fowler (67) is on 10 under, while two-time Masters champion Bubba Watson (68) is a shot further back.

Businessman Dimitri Kovtun told a news conference in Moscow he thought the death was "suicide by negligence".
Mr Litvinenko, 43, died three weeks after drinking tea laced with the radioactive substance polonium-210.
English prosecutors suspect Mr Kovtun and Andrei Lugovoi of poisoning him at a London hotel in November 2006.
Before he died, Mr Litvinenko - a former officer with the Federal Security Service (FSB), the successor to the KGB - accused Russian president Vladimir Putin of ordering his assassination. The claims were denied by the Kremlin.
However, speaking at a news conference in Moscow, Mr Kovtun said he believed Mr Litvinenko may have poisoned himself.
"I am more than sure that he dealt with polonium without knowing it," the Russian news agency Interfax reported him saying.
"Maybe it was leaking and polonium accumulated in his body gradually. It is possible that something he carried with him led to a gradual accumulation of polonium in the body," he added.
Mr Kovtun and former KGB bodyguard Mr Lugovoi have denied any involvement and have remained in Russia, despite an ongoing public inquiry into Mr Litvinenko's death.
Sir Ken Macdonald, who was then director of public prosecutions, had recommended in May 2007 that Mr Lugovoi should be charged with the murder of Mr Litvinenko.
Last month, Mr Kovtun offered to give evidence to the inquiry, at the Royal Courts of Justice in London, via video link.
Inquiry chairman Sir Robert Owen said he would grant Mr Kovtun "core participant" status to give evidence, if he met a number of conditions - including the provision of a witness statement and any relevant evidence.
Jordan Spieth leads the Tournament of Champions by four shots after shooting a nine-under second-round 64 in Hawaii.
[NEXT_CONCEPT]
Russian spy Alexander Litvinenko may have accidentally killed himself after handling radioactive material, a key suspect in his death has said.