Mathews made his T20 comeback for Sri Lanka in January after a three-year gap
Hamilton lapped the fastest circuit on the calendar in one minute 22.498 seconds, 0.047 quicker than Red Bull’s Sebastian Vettel.
Webber, second overall and three points behind the Briton with six races remaining, coasted to a standstill in the sunshine after nine laps and with 17 minutes remaining after suffering a complete loss of power in his Red Bull.
The Australian was also sidelined for much of Friday’s second session due to a water pressure problem.
The winner of four races this season, Webber removed his helmet and hitched a ride back to the garages on the back of a scooter.
Ferrari’s Fernando Alonso and Felipe Massa, who tracked through the gravel for the second session in a row, were third and fourth on the timing screens respectively for the Italian team’s home race.
McLaren’s world champion Jenson Button was fifth fastest, with the top five separated by just 0.226 seconds.
Hamilton and Button were running very different setups, with the former deciding against the ‘F-duct’ that stalls the rear wing and improves straight line speed.
The device offers less of an advantage on Monza’s long straights because cars have special rear wings designed for maximum aerodynamic effect.
However it does improve cornering and Button retained the duct on his car.
Red Bull were using a special fluorescent paint on the rear of their cars to help engineers monitor the air flow before deciding on a final set-up for qualifying.
Mathews made his T20 comeback for Sri Lanka in January after a three-year gap
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