Danny Willett is one stroke behind following a round of 64, with another Englishman, Chris Hanson, on 11 under.
Defending champion Rikard Karlberg of Sweden, who went round in 67, is tied for third with Hanson (65) and American Daniel Im (64) at Golf Club Milano.
England's Chris Wood is on 10 under and Scot Jamie McLeary a shot further back.
Rain and the threat of lightning saw play called off at 16:36 BST on Friday.
Karlberg, who led after a weather-delayed first round, faces the prospect of playing 36 holes on Saturday, having failed to start his second round on Friday.
Tournament director David Williams told the European Tour website: "If we had a dry night and some reasonable weather over the weekend, then we could still get four rounds in and finish on time on Sunday night."
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England's Chris Paisley leads the rain-delayed Italian Open by one shot after carding an eight-under-par 63 in the second round in Monza.