With two key games, AC Milan-Inter Milan and Napoli-Udinese, being held later on Sunday, Juventus had the chance to pile the pressure on closest challengers Napoli.
The Turin side got off to a great start when midfielder Andrea Pirlo gave them a 14th-minute lead from a sumptuous free-kick.
Siena had caused an upset at the San Siro a few weeks ago with victory over Inter, and they were given hope when Emanuele Calaio rose to meet Brazilian Angelo's cross to beat Gianluigi Buffon with a bullet header in the 47th minute.
Midfielder Claudio Marchisio, however, produced a superb volley from the edge of the area after the ball had been nodded out to give Juve an 85th-minute winner and extend their impressive unbeaten streak to 47 games.
Ahead of the evening fixtures, Juventus remain unbeaten on 19 points from six games, with Napoli in second on 16.
Klose scores brace
Lazio have already suffered two defeats although their visit to struggling Serie A newcomers Pescara was never likely to end in anything but a win for a side that had scored nine goals in six games.
Klose made it two when he latched onto Andrea Candreva's through ball in the 25th minute, and claimed his fifth league goal of the season ten minutes later when his header from a Hernanes corner beat the goalkeeper.
Elsewhere, embattled Cagliari's first victory of the season, a shock 1-0 win away to Torino, moved the Sardinians off the bottom thanks to Nene's early strike from the penalty spot.
Catania, meanwhile, maintained their solid start with a 2-0 win at home to struggling Parma, and Fiorentina should have scored more but claimed a precious 1-0 home win over Bologna.
Roma record win
USA midfielder Michael Bradley marked his recent return to action by scoring the second goal in Roma's 2-0 defeat of Atalanta at the Olympic stadium.
Argentinian midfielder Erik Lamela had put Roma ahead on the half-hour after a great through ball from playmaker Francesco Totti.
Roma's third win of the season moved them up to fifth while Atalanta's fourth reverse of the campaign dropped the Bergamo-based side one place to third-from-bottom.