Alonso moved to 10 goals for the season on Sunday after Nacional's 2-0 victory over Rentistas in Montevideo, two ahead of River Plate's Michael Santos, who also scored in his team's 2-2 draw with lowly Tacuarembo.
In the Uruguayan capital, Alonso showed his aerial prowess to score both goals in Nacional's victory, heading in a corner just before half-time, while he produced something spectacular for his second strike.
With 23 minutes left, Alonso burst into the area to meet Diego Arismendi's diagonal lofted pass, turning on the penalty spot to unleash a bicycle kick into the bottom corner of the net.
Nacional's win took them to 27 points after 10 games, five clear of second-placed Racing, who edged out Sud America 3-2 on Sunday.
Penarol (19 points) slipped from second to third after a 2-1 loss at Wanderers with Nicolas Albarracin scoring the winner in the 69th minute, while El Tanque Sisley (18) moved up to fourth with their second straight triumph - a 2-1 result over Defensor Sporting.
Rampla Juniors won 1-0 over Fenix, Atenas cruised past Cerro 3-1 and Danubio and Juventud completed a scoreless draw.
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