Filip Krajinović knocked out the sixth seed on a good day for Serbian tennis ©Getty Images

Novak Djokovic won his opening match of the Italian Open in routine fashion, but sixth seed Andrey Rublev was dumped out by Filip Krajinović.

World number one Djokovic broke Aslan Karatsev four times en route to a 6-3, 6-2 win on the Foro Italico clay in Rome.

Karatsev is Russian but allowed to compete as a neutral under Association of Tennis Professionals rules.

In another match pitting a Serbian against a player with no flag, Rublev was dispatched 6-2, 6-4 by unseeded Krajinović.

Krajinović needed only 69 minutes to beat the neutral Rublev, and has now won three of the pair's five meetings.

As top-eight seeds, Djokovic and Rublev both received byes into round two.

Canada's Denis Shapovalov had to play in round one yesterday, clashing with the umpire and crowd but still advancing, and today enjoyed a more serene win.

The Canadian, seeded 13th, beat Georgia's Nikoloz Basilashvili 6-4, 6-7.

Norway's Casper Ruud, seeded fifth, beat Dutchman Botic van de Zandschulp in three sets to also reach round three.

Jannik Sinner beat Spain's Pedro Martinez in straight sets to set up an all-Italian second-round clash with Fabio Fognini.

In the women's singles first round, US Open champion Emma Raducanu retired hurt with a back complaint.

The 19-year-old Briton was trailing Canada's Bianca Andreescu - herself a former US Open winner - 6-2, 2-1.

Tunisia's Ons Jabeur, fresh from winning the Madrid Open, dispatched Romania's Sorana Cîrstea in straight sets.