The company has offered to pay more than $4bn for its rival according to Reuters, which said Gardner Denver share had hit an eight-month high following the news.
Competing offers from KKR, Advent International and a partnership between TPG Capital and Onex Corp came in at between $3.6bn and $3.9bn, it added.
Previous reports have suggested SPX hopes to wrap a deal up by the end of the year, creating an industrial machinery conglomerate with a market value of about $7bn.
Gardner Denver was said to have cancelled management meetings scheduled with buyout firms this week, and did not even seek final offers before turning to SPX for one-on-one talks.
Blackstone and Bain Capital previously took part in the auction but did not make it through to the second round in a sale process which is expected to fetch about $3.5bn.
Gardner was founded in 1859 and makes industrial compressors, blowers, pumps and fuel systems.
The US-based business has 40 manufacturing facilities globally and posted revenues of $2.4bn last year, about two-thirds of which were generated abroad.
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