The Scot felt discomfort after completing his third round in the Constellation Senior Players Championship last week and took himself to Massachusetts General Hospital the following morning.
After undergoing tests and scans Montgomerie was able to return to the course in time to play his final round and went on to finish in a tie for third, nine shots behind winner Bernhard Langer.
And on Thursday the former Ryder Cup captain carded an opening 69 at Chambers Bay, matched by fellow 51-year-old Miguel Angel Jimenez, to lie four shots off the clubhouse lead of Henrik Stenson and Dustin Johnson.
“It feels great,” said Montgomerie, who qualified for his first US Open since 2008 by winning the senior equivalent last year.
“In practice I was shooting more like 80 but this morning the course was softer; they had to do that because they knew it would dry out.
“The greens were pretty receptive early on and allowed scoring. It’s just the length I have an issue with, especially off the tee.
“This is not a normal links course where you can run the ball into the greens. You have to carry the ball onto the surfaces and often that is right on my limit. So today a 69 was super. To break 70 was more than I was expecting.
“I wanted to represent the title I won last year and the position I hold. There’s a long way to go, but 69 is a good start. It’s nice to justify the exemption I was given.”
Montgomerie, who finished runner-up in the US Open in 1994, 1997 and 2006, added: “Can I win? Have stranger things happened?
“What Tom Watson did at Turnberry aged 59 (losing a play-off in the 2009 Open) has given everybody over 50 hope. So you never know. If the ball runs my way and I hole a lot of putts over the next three days, you never know. I could get into contention, which would be great.”