The pair, who arrived in Portugal during the close season, stole the show at a packed Alvalade stadium on a warm summer evening with two stunning goals.
The 26-year-old Montero, signed on loan from Major League Soccer club Seattle Sounders, headed the opener after 10 minutes, finishing off some neat flowing passing to take his tally to five goals at the top of league's scoring chart.
Sporting lead the standings with seven points from three matches while Benfica are sixth on four. Champions Porto, who won their first two matches, visit Pacos de Ferreira on Sunday.
Sporting controlled the first-half possession while Benfica looked some way off the commanding performances that led them to last season's Europa League final, which they lost to Chelsea.
But Markovic, who came on for the injured Eduardo Salvio, managed to pull Benfica level in the second half.
The 19-year-old's first mazy run ended with Spanish striker Rodrigo wasting a chance but Markovic made no mistake when he dashed past three defenders and slotted home after 65 minutes.
Sporting endured one of their worst campaigns last season but arrived at the derby in fine form, having won their first two league matches 5-1 and 4-0.
In contrast, title candidates Benfica lost their opener and scraped a 2-1 win at home to Gil Vicente last weekend with two stoppage time goals.
(Editing by Ken Ferris)
