Sosa hit his 63rd homer in the eighth inning and drove in all of Chicago's runs as the Cubs beat the San Diego Padres 6-3 on Wednesday night.
McGwire and Sosa now have nine games left to bring their duel to a conclusion - the prize being sporting immortality.
With flashbulbs popping and most of the crowd of 49,891 on their feet, Sosa drove a 93 mph fastball from Brian Boehringer into the second deck in left field with two outs. Sosa had drawn level on 62 with his St Louis Cardinals rival, McGwire, on Sunday when he connected twice in a stunning performance at Wrigley Field. That gave him four homers in three days. Not to be outdone, McGwire went ahead again when he hit his 63rd in the ninth inning of the first game of a double-header on Tuesday night.
Then, on Wednesday, the crowd went wild when Sosa's ball settled inside the foul pole, setting off a mad scramble for the souvenir. An adoring public demanded that he was summoned back on to the field for a curtain call. Mark Grace stepped out of the batter's box and Boehringer stepped off the mound during the ovation.