Monday's first game saw Aston Villa beat Sheffield United 1-0 in Villa Park on a 63rd minute goal by Ezri Konsa Ngoyo.
Leo Messi will also be in the spotlight after a summer in which he asked to leave the Camp Nou, while the pressure will also be ramped up on Antoine Griezmann and Philippe Coutinho, who has returned following his loan to Bayern Munich.
Selby, 37, began the first stage by beating Fergal O'Brien 3-1 in the four-man group. Then the world number four defeated Brandon Sargeant with the same scoreline before drawing 2-2 with Chinese player Lyu Haotian in the last match.
Martinez's side are tight in midfield and defense, really dangerous from set pieces and have the experienced Roberto Soldado in attack; this week they have shown last season was no fluke. Enditem
The only down side for the capital-based team was that Renato Augusto limped off with injury as the Brazilian was suspected of suffering a muscular problem.
Mainz were unable to put up resistance but were lucky enough that Forsberg rattled only the woodwork in the 31st minute and Angelino hit the side netting from tight angle before the half time.
The top four teams at the end of the qualifying tournament will automatically earn a World Cup berth. The fifth-ranked side will advance to an intercontinental playoff over two legs.
Li made four straight points from three kills and one block to start the second set before Zhang spearheaded Jiangsu to fight back with a 7-0 run.
Hours later, Jiangsu Suning regained the second place after playing out a 0-0 stalemate with Shanghai Shenhua.
The growing number of fans returning their tickets is an ominous sign for both sides.
Brighton meanwhile impressed with a 3-0 thrashing of Newcastle United and the game is already looking like a watershed moment for Ole Gunnar Solskjaer, who needs a quick reaction in order to calm jittery Old Trafford nerves.