Nico Rosberg's Mercedes engine suffered a temperature-related problem, which gave him less power than Formula 1 team-mate Lewis Hamilton at the start of the Japanese GP, says Toto Wolff.
The German got away cleanly off the line from pole, but in the next phase of the start Hamilton was able to get alongside through Turns 1 and 2 to complete a pass for the lead.
Hamilton went on to claim his eighth win of the season with Rosberg second as the reigning world champion extended his lead in the drivers' standings to 48 points with a maximum of 125 remaining.
"The initial getaway was good for both cars but Nico had a little bit of an issue with a hotter power unit," said Mercedes boss Wolff.
"When it kicked in after a couple of seconds, he didn't have the same power as Lewis.
"This is temperature-related. That was showing up on the formation lap."
Wolff said Mercedes is unsure what caused the problem but he suggested it could be down to the way the driver prepares the clutch and engine on the formation lap.
"It is important to get the clutch and engine into the right temperature window," he said.
"The driver manages that in the installation lap. So they are different procedures.
"Both of the drivers have worked on that. Today, that lap triggered different temperatures in the power units."
Rosberg closed on Hamilton in the closing stages, but Wolff said Hamilton was controlling his pace to get the car to the chequered flag.
"Lewis was managing the gap," said Wolff "We had some minor drama with the car - temperature-related issues - and Lewis flat-spotted his tyre and that caused massive vibrations.
"When we took the tyre off the car, it was flat-spotted down to the carcass so that could have ended badly.
"We tweaked the power unit and chassis a lot because we were seeing signs of reliability issues which worried us a bit."