It happened about 1 pm Sunday afternoon at the intersection of North Point and Taylor.
"He looked pretty terrible, eyes closed, he looked like he was in pain," witness Brandon Kaleikini told KTVU, "and I was wondering if he was going to make it out alive."
The unidentified cyclist, a 44-year-old man, was hospitalized with a punctured lung and broken collarbone.
"I saw that he was breathing very very heavy" described Kaleikini,"he was just out of it."
The Uber driver, identified by police as 38-year-old Emerson Decarvalho of Daly City.
Witnesses heard the two men shouting at each other just east of the intersection, where a bike lane runs parallel to the traffic lane.
Police aren't sure where or why the conflict began, but the antagonism was mutual.
"The cyclist, from witness accounts, was banging on the Uber cab's window and pushed in his side view mirror," SFPD Capt.David Lazar told KTVU, "then they both picked up speed and the car swerved, hitting the bike's rear wheel."
Police say Decarvalho remained at the scene, was questioned, then arrested for Assault with a Deadly Weapon, his car.
"The cyclist fell off his bicycle and landed on his face," elaborated Captain Lazar, "and we believe he may have lost consciousness, then awakened by the time we arrived."
"I saw them put him onto the stretcher and then wheel him away, he didn't look too great, he was turning blue," witness Nicole Brooks told KTVU, noting how cars and bicycles are squeezed together, even with a dedicated bike lane.
"I've seen people hit cars, I've seen drivers get out and threaten, and fight, it happens all the time down here, and it sucks."
Uber, in a statement, says it is "gathering more information" and "stands ready to assist" in any way it can.
The company was unable to provide any background on Decarvalho but added "Our thoughts are with the victim of this terrible incident."
Uber has about twenty thousand drivers, which it refers to as "partners", in the Bay Area.