Following a lengthy All-Star break, during which the National League blanked the American League in the annual gathering of the sport's top players, pennant races will begin in earnest.
The Yankees have opened the largest division lead of all with their 52-33 record, owning a seven-game advantage over Baltimore in the AL East.
The Chicago White Sox are out front by three games over Cleveland in the AL Central and two-time defending league champion Texas has a four-game cushion in the AL West over the Los Angeles Angels.
Washington, with its strong pitching staff, has built a 49-34 record in the NL East and leads Atlanta by four games while the Los Angeles Dodgers lead San Francisco by 1/2 game in the NL West.
It is in the NL Central, however, where the biggest shock has occurred. Pittsburgh has not had a winning season in two decades, but the Pirates are 11 games above .500 heading into the second half and are in front of Cincinnati by a game.
Detroit will take baseball's longest active winning streak into Friday's action, having won five in a row.
The Yankees will host the Angels in one of Friday's top attractions while Pittsburgh will be at Milwaukee.
Pittsburgh has been led by Andrew McCutchen, whose .362 batting average tops the majors. His 18 home runs are six behind Milwaukee's Ryan Braun for the league lead and his 60 RBI trail league leader Carlos Beltran of St. Louis by five.
Josh Hamilton of Texas is tied with Toronto's Jose Bautista for most homers in the majors with 27 and Hamilton's 75 RBI leads all of baseball. The American League batting leader, however, is the Angels' Mike Trout. He will carry a .341 average into Friday's game with the Yankees.
R.A. Dickey of the New York Mets is the winningest pitcher in the sport with 12 victories. The Angels' Jered Weaver is No. 1 in ERA at 1.96 and Jim Johnson of Baltimore leads all closers with 26 saves.