Clark had his first career two-goal game and Brett McLean also scored for the last-place Avalanche, who had lost three of four before handing the Sharks their first home loss in five weeks. Jose Theodore made 28 saves in his 150th career victory.
Joe Thornton and Jonathan Cheechoo scored for the Sharks, who weren't sharp in their return from a strong four-game road trip. San Jose, which played just 11 home games in the season's first nine weeks, began a stretch with 12 of 15 games at the Shark Tank.
Vesa Toskala made 20 saves for San Jose, but lost for just the third time in 16 games this season when he couldn't stop McLean's go-ahead goal and Clark's second score 5:04 apart late in the second period.
Sakic, who has 65 points in 47 career games against the Sharks, added his 11th goal of the season in the third.