The 1993 GGCC champion started the third round one behind co-leaders Calcavecchia and Robert Gamez, but birdied his first three holes at Forest Oaks Country Club to tie for the lead.
Mediate bogeyed his next two holes to fall from the top; however, he rebounded on the back nine with three birdies in four holes. The four-time PGA Tour winner finished his scoring with a 20-foot birdie at the last to reach 15-under.
Mediate, ranked 140th on tour in putting average, needed just 25 swipes of the flatstick in his third round and 77 so far after 54 holes.
'I've worked on a few things with my right-hand grip and slowed my pace down a lot - you know, things that we as professionals change every other day,' Mediate said. 'I don't know why I hadn't been putting good. I think my pace got off and my lines got crooked. That's all you need.
'There were no putts that really got away from me today. I just had a little bit better feel.'
Like Mediate, Calcavecchia also birdied the 18th to shoot 69 and get to 13-under-par.
Chad Campbell is two more shots back at 11-under after a 66, while Gamez, who hasn't won in 12 years, stumbled to a 73 and is in a fourth-place tie with Jonathan Kaye, six shots behind Mediate.
Campbell, who earned a battlefield promotion last year by winning three times on the Buy.Com Tour, birdied four of his first five holes to card and 30 on the front side, then closed out his round with a tap-in birdie on No. 18.
'Every time you get a chance to win, no matter where you're playing, it definitely helps,' Campbell said. 'Obviously, it's a little different out here, the players are the best in the world. But all the times you get up there you learn something different and it makes you feel a little more comfortable.'
Kaye, who is seeking his first win in five-plus seasons on tour, got into the thick of things on the back nine when he birdied Nos. 10, 13 and 14 and eagled 15 to get to 12-under.
But Kaye, who was cruising to a 63, had to settled for a 66 when he double-bogeyed the par-3 17th and bogeyed the 18th.
Also of note, officials said Saturday the tournament's name will be changed to the Chrysler Classic of Greensboro when the event moves to the fall in 2003. The purse also will increase $700,000 to $4.5 million.
Full-field scores from the Greater Greensboro Chrysler Classic