Hunter Pence also collected three hits off the three-time Cy Young Award winner, who improved his lifetime record against the Giants to 22-9 with a 1.60 ERA. In one afternoon, Pence and Williamson matched the number of three-hit games Giants hitters had totaled against Kershaw in his 39 previous starts against the team. Those belonged to Buster Posey (Sept. 2, 2015) and Joaquin Arias (April 28, 2015).
Said right-hander Chris Stratton, the unfortunate soul who had the dubious privilege of opposing Kershaw, "You definitely know that if you get one or two runs off him, it's a good day."
It was a particularly good day for Williamson, 27, who's still striving to establish himself as a Major Leaguer after receiving intermittent stints with the Giants since 2015.
"I'm trying to leave a good impression going into the winter," Williamson said.
Williamson, who's batting .242 in 25 games, might have had a legitimate chance to raise his profile early in the season. But a strained left quadriceps forced him to open the campaign on the disabled list. Injuries have been an issue for Williamson, who joined the ranks of nonpitchers to undergo Tommy John surgery in 2014.
"Injuries set you back with your development," Giants manager Bruce Bochy said. In Williamson's case, Bochy added, "It's not a question of the talent or the power. It's just a question of getting consistent with it and making it usable."
When healthy, Williamson's capable of prodigious efforts, as his homer off Kershaw indicated. According to Statcast™, Williamson connected with a 92.4 mph fastball and drove it 410 feet over the center-field barrier at a rate of 106.3 mph.
"That shows how strong he is," Bochy said. "It looks like his confidence is growing."
Williamson also asserted himself defensively, making two strong throws from left field. Curtis Granderson barely beat Williamson's first throw with a feet-first slide in the second inning; a sixth-inning peg apprehended Yasmani Grandal at third base.
But Sunday's deeds won't necessarily guarantee Williamson a clear path to a big league role. Sandwiched on the Giants' theoretical depth chart by veterans (Pence, Denard Span, Gorkys Hernandez, Jarrett Parker and anybody the team might acquire in the offseason) and Minor Leaguers (Chris Shaw, Bryan Reynolds), Williamson must prove he can regularly repeat such performances.
"It's all about competing," Bochy said. "We're going to be open-minded in Spring Training -- I think you have to be, when you see where we are -- and there are going to be guys vying for jobs. Tha's the way it should be."
Chris Haft has covered the Giants since 2005, and for MLB.com since 2007. Follow him on Twitter at @sfgiantsbeat and listen to his podcast. This story was not subject to the approval of Major League Baseball or its clubs.