Western Sydney Wanderers coach Tony Popovic has warned Youssouf Hersi's dazzling A-League form is yet to peak because he's still not fully match fit.The Ethiopian-born Dutchman continued his stellar season with a double strike in the Wanderers' 4-2 win over Adelaide at Hindmarsh Stadium on Sunday.The result extended the rookie club's winning streak to a record four games and Hersi has been instrumental in that run with his pace and trickery.Hersi did not play for almost a year and a half before joining the Wanderers, with his last match for Dutch club De Graafschap played in February, 2011.And while Popovic is reluctant to single Hersi out for praise, he believes the midfielder's best to yet to come as the finals approach, a worrying prospect for A-League rivals who have already been tormented by the 30-year-old."There's more to come from him," Popovic said on Monday."He's a player that didn't play any football for almost two years."He still struggles to finish 90 minutes, so there's a lot more to come from him."Like the other foreign players, we're pleased with his progress but I wouldn't single him out any more than any of the others."Hersi is yet to be re-signed for next season and Popovic was coy when asked about a potential contract extension."We're speaking to all the players we want to re-sign for next year but one performance doesn't make that more of a priority for me," he said.Mateo Poljak is another of the Wanderers' foreign legion who continues to impress, scoring his first A-League goal in the win over the Reds.With Western Sydney sitting just four points off the competition lead, the Croatian defender admitted to some surprise at the fact the side was challenging for the title in its first season.But he believes anything is possible, if the Wanderers don't get ahead of themselves as the finals approach."There are still a lot of games to come before the playoffs and we have to take it game by game," Poljak said."But if we continue like this, I think we have a chance."The Wanderers host Newcastle Jets at Campbelltown Stadium on Saturday, with next week's showdown with Melbourne Victory looming large.