Out since May 14 with a fractured right leg, the big righty threw a three-inning simulated game prior to Tampa Bay's game against the Orioles.
"Even after the off-day and the long day of travel the day before, I still felt good," Niemann said. "It's really less and less in my mind each day. We're starting to think more [about] what we're doing on the mound. It's a great transition to not worry about how you feel."
Niemann, who is scheduled to throw once every five days, said he used each of his pitches. He hinted at the desire to make a rehab start, possibly for Class A Charlotte, but would be fine throwing another simulated game.
Rays manager Joe Maddon also hinted that Niemann making a rehab start in the near future is a possibility. Whenever Niemann does take the mound in a game, he expects it'll take about 100 pitches in an outing to be ready to return to the Majors, and he could be ready in as few as four starts.
Another player who could provide a big boost for the club is third baseman Evan Longoria, who is on a rehab assignment with Triple-A Durham. In five games for the Bulls, Longoria is 3-for-15 with five walks as a designated hitter.
Longoria, who has been sidelined since April 30 with a partially torn left hamstring, has a hit in each of his last three games. Maddon said Longoria could return to the Rays during the team's upcoming series against the Blue Jays, which begins on Tuesday, but he must also take his time.
"He's basically gonna be coming out of Spring Training with 1 1/2 legs," Maddon said. "You gotta be patient with all that and understand there might be a little bit of a curve to be worked with. I'm prepared for that mentally, but of course we want him back."