Farnesi-Vini rider Guardini won the sprint at the end of the rain-soaked 100.8-kilometre stage from Pekan to Chukai.
The Italian crossed the line fractions of a second ahead of home rider Harrif Salleh (Terengganu) and American Jake Keough (UnitedHealthcare), who joined him on the podium.
That victory draws Guardini level with Australia's Graeme Brown as the most prolific stage-winner in the race's history.
Serpa (Androni Giocattoli-Venezuela) retains a 30-second lead over Jose Rujano in the general classification over team-mate Jose Rujano.
- Related links