Power was quick to go to the top of the timesheets with Scott Dixon and Sebastien Bourdais his closest competition for much of Saturday's practice, although James Hinchcliffe put in a flier to briefly go top halfway through the 60 minute session with a 1:21.9137s.
That was the fastest time by anyone on the street course up to that point in the weekend as the track continued to rubber in and got faster and faster throughout the hour, and Hinchcliffe ended up being pushed down to eighth by the end of the session despite improving his time by another quarter of a second in the meantime.
Dario Franchitti was slow to start putting in competitive fast laps and only finally popped into second spot with ten minutes remaining, albeit still 0.2217s off Power's best time of 1:21.0176s. Graham Rahal emerged in third place edging out Dixon for the spot in the final minutes.
The IndyCar drivers continued to learn the brand new street course in Baltimore, with Ed Carpenter, Helio Castroneves and Takuma Sato simply the first of many to repeatedly run off at turn 1 as drivers had extreme difficulty identifying the correct turn-in point for this corner.
Alex Tagliani was the first driver to trigger a full course caution when he ran off at the double chicane turn 6 after missing the left-hander, the #77's engine then dying when he tried backing up.
Marco Andretti had a more serious accident shortly after the midpoint of the session when he got unsettled over the kerbing into turn 8 and made contact with the concrete wall, heavily damaging the front end of the #26 and bringing out the second full course caution. Marco himself was quick to jump out of the cockpit and was unharmed, but took no further part in the session.
Martin Plowman suffered a flat left front tyre 12 minutes before the end of the session but was able to recover to pit lane without incident or causing a caution. Mike Conway locked up and had light contact with the wall as the session came to an end, which also saw a late spin for Scott Dixon.
The threat of rain was in the air as the clouds rolled in, with a line of thunderstorms threatening to track into the area in time for qualifying scheduled to commence at 12.05pm (5.05pm BST). Unlike Friday's disruption for late-running track preparations, there were no changes or delays to report for the start of the Saturday schedule.
Qualifying will take the street/road course format starting with the field split into two 14-car groups. The top six times from each progress through to the next round, with the top six from round 2 then entered into the Firestone Fast Six pole shoot-out.