United States lock Hayden Smith has no regrets about his decision to swap NFL in favour of a return to rugby union.

Smith is competing at the Rugby World Cup in England/Wales, three years on from his stint in the NFL, and featured as USA lost their tournament opener against Samoa on Sunday.

The 30-year-old Australia-born utility, who initially played basketball through college, tried his hand at American Football in 2012 after running out for USA in the World Cup a year prior, recruited as a tight end by the New York Jets.

Smith returned to rugby and Saracens in December 2013, and the 27-cap USA international is loving life back in the 15-man sport.

"It's fantastic. One thing I certainly missed in the NFL is the culture of rugby, what it's like to be around the guys," Smith told reporters.

"It's harder to have that in the NFL with the way the setup is, new guys coming in all the time, new coaches. It's harder to build that spirit."

Smith and USA face Scotland in their second Pool B fixture at Elland Road on Sunday.