FORTUNE — It’s a boast you don’t hear too often from CEOs: “I’m doing it for sport.” Yet that’s precisely what Keith Krach says drove him last year to become chief executive of DocuSign, a company that offers electronic-signature technology for transactions and document verification. “I never thought I’d go back and do the CEO thing again,” Krach says, “but this is a great, high-performance team. It’s a fun group of people. We all love to win.”
Krach can afford, literally, to be a bit flip about his reasons for taking the DocuSign gig. The last time he did “the CEO thing,” he was heading up Ariba, the business-to-business marketplace he co-founded in 1996. The company reached a peak value of $34 billion in the dotcom boom and this year agreed to sell to SAP (SAP) for $4.3 billion.
Krach, who at 55 is something of an elder statesman in Silicon Valley, left Ariba in 2001. He focused on philanthropy and took his eighth-grade son out of school for a year of adventures that included climbing Mount Kilimanjaro and building homes for Habitat for Humanity in Honduras. Krach got back into the tech game gradually, investing in startups and taking board seats. After two years as DocuSign’s chairman, the opportunity to become CEO last year proved too good to pass up.
“This is like Ariba on steroids,” he says. “Every company and every department in a company is a potential customer. Every individual is a potential user. By the second half of this decade, the vast majority of signatures are going to be electronic.”
Indeed the rise of smartphones and tablets and the growing use of such devices for transactions (transmitting contracts and legal documents) translate into big opportunities for DocuSign, which recently nabbed a $55 million investment from a group led by Kleiner Perkins Caufield & Byers. Krach declined to disclose revenue, but DocuSign now serves more than 20 million customers.
It’s easy to see why companies are embracing e-signatures. At Salesforce.com (CRM), for example, it used to take two days to go from a verbal customer agreement to a signed deal. “The sales guys would be pulling their hair out,” says Jim Steele, chief customer officer of Salesforce, which is an investor in DocuSign. Now that Salesforce uses DocuSign, 90% of deals are completed in less than a day and 60% take less than 15 minutes, Steele adds. The service is free for signers, but businesses that send out documents for signatures pay monthly fees that start at $14.99 per user.
As the leader in the market, DocuSign faces increased competition from the likes of Adobe and from startups like HelloSign. DocuSign has responded with a consumer app, DocuSign Ink, which Apple (AAPL) selected as one of the best new apps of 2011. Its biggest competitor remains old-fashioned paper. Says Krach: “Once you use it, you never want to go back to the old way of doing things.” Unless, of course, you’re doing it for sport.
This story is from the October 8, 2012 issue of Fortune.