Mercer never hesitated rounding second, but Kelly got the ball in to shortstop Eugenio Suarez in time for a play at third. Castellanos collected the ball and made a tag as Mercer slid his lead foot into the bag.
Third-base umpire Alfonso Marquez ruled Mercer safe. Ausmus waited for a second, then emerged to challenge, taking a shot that he could erase a baserunner for Farmer.
Replays showed the play was close, and different angles made differing cases, but the most definitive suggested Mercer got his foot in at the same time as Castellanos' tag hit his leg. After a 2-minute, 54-second review, umpires ruled that the call stands, meaning there wasn't sufficient evidence to overturn the call.
The call set up an extra run once Travis Snider hit Farmer's next pitch out to right for a two-run homer and a 4-1 Pirates lead.
Ausmus fell to 17-for-29 in replay challenges this season.
Jason Beck is a reporter for MLB.com. Read Beck's Blog and follow him on Twitter @beckjason. This story was not subject to the approval of Major League Baseball or its clubs.