And Rivers bit.
"Maybe he was expecting me to go up the seam," Alonso said.
Instead, Alonso let a receiver pass through his zone before settling in on wide receiver Tyrell Williams. Alonso then intercepted a pass intended for Williams and returned it for a 60-yard touchdown, lifting Miami past San Diego 31-24.
Alonso collected his first defensive touchdown -- including high school and college.
"We just kept battling, man," Alonso said. "We were in some bad situations, but we got out of them."
Miami coach Adam Gase wasn't surprised Alonso delivered.
"He's a very instinctive player, and there are a couple of times that he gambles and loses," Gase said. "But a lot of those times he gambles and he is right."
Tony Lippett's second interception of the game in the closing minute clinched the victory for Miami (5-4).
"Obviously, you can't let that happen," Rivers said of his four fourth-quarter turnovers. "On two of them, we were fixin' to score. And with the last one, we had a chance to win the game with the field goal."
Andrew Franks pulled the Dolphins even, 24-24, with a 27-yard field goal with 2:57 left.
A 51-yard, Rivers-to-Williams touchdown pass had pushed the Chargers ahead 24-21 with 4:04 to play. It was Rivers' third touchdown pass of the game and 301st of his career, which eclipsed John Elway for No. 8 on the all-time list.
Rivers was 23 of 44 for 326 yards with four interceptions as the Chargers (4-6) failed to reach .500 for the first time since Week 2.
The Chargers had made it a point of emphasis to sweep their two-game homestand, and they beat the Tennessee Titans last week. Where does the loss to the Dolphins leave San Diego?
"At 4-6," a glum coach Mike McCoy said. "We fell short and didn't get to it. We had an opportunity to get to 5-5, and that was important for all of us going into the game."
Miami's Ryan Tannehill completed 17 of 24 passes for 240 yards and two touchdowns with no interceptions while leading the Dolphins to their fourth consecutive win. Jay Ajayi rushed for 79 yards on 19 carries, breaking his three-game streak of 100-yard rushing outings.
Chargers running back Melvin Gordon, who rushed for 196 yards last week, was restricted to 70 yards on 24 carries.
Miami's Jakeem Grant committed a huge blunder to open the fourth by fumbling a punt at the Chargers 5, and with San Diego's Darrell Stuckey making the recovery. However, the Chargers failed to take advantage.
Rivers tried forcing a pass into double coverage to Williams, and it was intercepted by Lippett.
Dolphins running back Damien Williams scored his second touchdown when he corralled Tannehill's 18-yard pass late in the third quarter. Tannehill's read took him to Damien Williams, who was being covered by inside linebacker Kyle Emanuel, and Williams easily raced past Emanuel in putting the Dolphins up 21-17.
Rivers threw his second touchdown pass to a tight end when connecting with rookie Hunter Henry as the Chargers surged ahead 14-10 late in the third quarter. Flushed right from the pocket, Rivers saw Henry streaking across the end zone for a 7-yard completion.
Miami took its first lead of the game at 14-10 when Damien Williams rushed in from the 2 to open the third period. Ajayi had the big play on the drive when he carried for 40 yards to get into Chargers territory. Ajayi was a decoy floating out to the left side on Williams' scoring run up the middle.
The Dolphins pulled within 10-7 with Tannehill threw a strike to Kenny Stills from 39 yards. Dwight Lowery had tight single coverage, but Stills made a diving catch late in the second quarter.
Antonio Gates caught the 109th touchdown pass of his career midway through the second quarter, pushing the Chargers ahead 10-0. Gates, who grabbed an 11-yard Rivers pass, is two touchdown catches shy of matching Tony Gonzalez for the most by a tight end in NFL history. It was Rivers' 82nd TD toss to Gates.
After a scoreless first quarter, the Chargers took a 3-0 edge on Josh Lambo's 45-yard field goal, capping a seven-play, 46-yard drive that was helped by a key Miami penalty.
After dropping four of their first five games, the Dolphins haven't lost since Oct. 9.
"I think we are starting to show success in all three phases of the game," Miami tackle Branden Albert said. "Everyone is feeding off each other."
NOTES: The Dolphins activated DT Earl Mitchell from injured reserve. ... Miami will remain in southern California this week. The team will practice in San Diego County in the days leading up to next Sunday's game at the Los Angeles Rams. ... San Diego WR Tyrell Williams started in place of Travis Benjamin (knee), and he finished with five catches for 125 yards. ... Miami WR DeVante Parker caught five passes for 103 yards. ... The Chargers have their bye next week before playing the Texans in Houston on Nov. 27.