The Kings improved to 6-2-1 in their last nine games, a winning stretch that came on the heels of a 1-9-1 skid that spanned from Jan. 11 to Feb. 9.
Los Angeles, which is in last place in the Pacific Division, snapped Vegas' franchise record-tying eight-game win streak. The Golden Knights hadn't lost since a 4-0 defeat in Minnesota on Feb. 11.
''Obviously, they were kind of hot, so we wanted to come in here ... and have a good performance,'' Petersen said, ''and I think everybody did their job.''
It marked the third straight game Golden Knights fans were treated to an exceptional goaltending performance, after Marc-Andre Fleury registered the 61st shutout of his career and Robin Lehner won his debut with Vegas. This time, however, the announced gathering of 18,419 watched Petersen improve to 2-0-0 against Vegas.
Petersen was brilliant in stifling the Golden Knights' normally potent offense that clearly was missing Mark Stone, who is week-to-week with a lower-body injury. Of his 42 stops, 19 came in the third period.
''I made the saves I needed to make, and it worked out,'' Petersen said.
It was the second straight game Petersen held a playoff-contender to just one goal, after beating Pittsburgh on Wednesday, when he stopped 36 shots. The 25-year-old was making just his sixth start of the season - all since Feb. 8.
''We're playing for pride and we're playing for jobs next year, especially for the young guys,'' said Petersen, who is 3-3-0 with a 2.68 goals-against average and .926 save percentage this season. ''It's a great opportunity to get games in and play an important role. We're trying to establish a foundation for next year and years to come, so I think it's a really important time for us.''
Trevor Lewis and Alex Iafallo also scored for the Kings.
Shea Theodore scored for Vegas, while Marc-Andre Fleury made 13 saves.
Fleury came in riding a 5-0-0 win streak since Feb. 15, along with a 1.60 goals-against average and a .942 save percentage, but Kopitar cooled off the 16-year-veteran netminder by putting the Kings on top with two first-period goals, including one on the power play.
''They're a good home team and any time you can get on the board first, it's beneficial,'' said Kopitar, who leads the team in goals (20), assists (39) and points (59). ''We've played some good hockey lately. There's certainly room for improvement. Tonight, Cal was outstanding, obviously. We were able to beat them on special teams, so that's always huge.''
The Kings were 2 for 2 with a power-play unit that came in ranked 24th in the league, but victimized a Vegas team that entered with the fourth-worst penalty kill on home ice.
After Lewis' nifty spin move around Vegas defenseman Nick Holden beat Fleury far side over his glove to make it 3-0 midway through the second, Vegas got its first goal late in the period. Max Pacioretty clanked a blistering slap shot off the post that rang as loud as any shot ever taken at T-Mobile Arena. Los Angeles' Ben Hutton tried clearing the rebound, but Theodore intercepted the puck in the high slot and beat Petersen before he could recover.
After former King Alec Martinez was called for goaltender interference, LA capitalized on the power play again. After stopping Adrian Kempe's slapper from the right circle, Fleury couldn't control the rebound and Iafallo cleaned up the rebound to push Los Angeles' lead back to three goals at 4-1.
It was a stark difference from the Golden Knights who finished the month of February with a 10-2-1 record and an .808 points percentage that led the NHL.
''I don't know if it was kind of lackadaisical just because we've been winning, but we can't think like that,'' Vegas forward Paul Stastny said. ''We can't look at what we've done in the last couple of weeks. I think our mindset's got to be every game, no matter who we're playing, whether we're playing a top team in the West or a bottom team in the East or vice-versa.
''These points are so important for us.''
NOTES: Stone, tied with Pacioretty with a team-high 63 points, missed his second straight game. Vegas coach Peter DeBoer said Stone will not need surgery and hopes to have the forward back by the end of the regular season. ... Carrier is one game shy of playing in the 200th of his career. ... Vegas is 12-4-2 since replacing Gerard Gallant with DeBoer.
Kings: Open a nine-game homestand vs. Toronto on Thursday.
Golden Knights: Conclude a four-game homestand vs. New Jersey on Tuesday.
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