Mercedes driver Lewis Hamilton beat Ferrari's Fernando Alonso in final practice at the Italian Grand Prix as Nico Rosberg hit trouble.
Hamilton's team-mate and title rival suffered a gearbox problem that meant he missed almost the entire session.
Mercedes executive technical director Paddy Lowe said he was confident the German would not suffer a five-place grid penalty for changing his gearbox.
Rosberg currently leads Hamilton by 29 points in the drivers' title.
Hamilton, who had his own problems on Friday, was 0.412secs clear of Alonso.
Ferrari's pace was impressive and unexpected - the Spaniard himself had said heading into the race this would be one of the team's most difficult races of the season because of the long straights at Monza.
Alonso's time came on a lap that was beautiful in its controlled aggression, balance and delicacy, the Ferrari drifting through Lesmo 1, and flirting with a penalty by running right out to the allowed limit through the iconic final corner, Parabolica.
Hamilton appeared controlled and comfortable, but needed a second lap on the faster medium tyres to set his quickest time, and beat the lap he set earlier on the slower hard tyre, which was itself 0.189secs quicker than Alonso's best time.
"Lewis had a good run on the prime and the option tyres so we're reasonably confident we can have a good result this afternoon," Lowe said.
Alonso was out of his car before the end of the session and was embraced in the Ferrari garage by the company's president Luca Di Montezemolo, whose future is the subject of intense speculation.
The Italian media is reporting that Di Montezemolo, who has led Ferrari since 1991, is to leave his post to run the airline Alitalia.
But even that scenario is overshadowed by the continuing tension at Mercedes, following Rosberg's apology for colliding with Hamilton at the last race in Belgium.
Mercedes have told both drivers that they can continue to race freely but must not collide, and have been warned by team boss Toto Wolff that if they cannot be managed they could be sacked.
Both drivers have now lost time this weekend - Hamilton lost two-thirds of Friday's second session with an electronics problem and Rosberg a similar amount of time with the gearbox electronics issue on Saturday morning.
Lowe said it was difficult to say which driver would be most affected by the lost time, but added: "Probably today is slightly more damaging than yesterday."
Williams driver Valtteri Bottas was third, 0.571secs behind Hamilton, followed by team-mate Felipe Massa, McLaren's Jenson Button, Red Bull's Sebastian Vettel and Ferrari's Kimi Raikkonen.
The two Ferraris in second and seventh places were separated by just 0.4secs. "It's very close, isn't it?" said Williams chief technical officer Pat Symonds. "We expected to be competitive here."
Symonds said he did not think Williams could beat the Mercedes in qualifying. "We're shooting for the second row," he said.