Johnson, who arrived in Houston after a US Davis Cup tie in Australia, won his second career ATP title, and his first on home soil.

"To win on US soil and win when you're not feeling great is a testament to my willpower and competitiveness," he said.

"My body has just physically run out of gas after this week. Coming in from Australia kind of put me behind the eight ball and then the nerves of trying to close out the final, a lot of factors go into cramping. I was very fortunate to get out of that."

Eighth-seeded Bellucci was vying to become the first player since 2004 to win five three-set matches en route to an ATP singles title.

"I wasn't cramping, but I was so tired that I couldn't think to win the match," he said. "He was a little bit worse than me (physically), but he was playing unbelievably."

But Johnson was quickly down in the third set, surrendering an opening service game that included back-to-back double faults.

As Johnson's legs began to cramp as he trailed 4-2, it appeared the American was on his way out.

In a bid to keep the points short Johnson loaded up his forehands. He gave himself a break chance with a passing winner and broke Bellucci on his way to levelling the set at 4-4.

Although the cramps intensified, Johnson held on to force the tiebreaker in which he built a 6/3 lead. Bellucci saved two match points before Johnson sealed the win.

"I'm a little bit sad because I think I could have won," Bellucci said. "But I'm happy to make another final."