Phelps shares the honors in Santa Clara
Michael Phelps competes in the men's 100 meter butterfly preliminaries during the 2014 Arena Grand Prix of Santa Clara at the George F. Haines International Swim Center on June 20, 2014 in Santa Clara, California - by Ezra Shaw
Phelps, who came out of a near two-year retirement in April in a comeback that could take him all the way to the 2016 Rio Games, was second to Shields at the turn.
The 18-time Olympic gold medalist closed the gap, and they were matched stroke-for-stroke in the final 25 meters before hitting the wall in the same time of 52.11sec.
Phelps's evening also included a runner-up finish to Olympic champion Nathan Adrian in the 100m freestyle.
Adrian clocked a blistering 48.17sec. In an event that has never been one of his signature races, Phelps was second in 48.80 with Connor Dwyer third in 49.76.
Although it wasn't close, Phelps has achieved the time standard needed to contest the 100m free at the US championships in August, the selection meeting for the Pan Pacific championships and next year's World Championships in Russia.
Phelps, whose glittering Olympic career included his stunning eight gold medals in eight events at Beijing in 2008, has yet to confirm he has his sights set on the 2015 worlds or the Rio Games.
But coming off of two weeks of high-altitude training, the 28-year-old is showing in the third meet of his comeback that he's ready to challenge himself.
Friday marked the first full double-duty day -- preliminaries and finals of two events -- of his comeback.
In earlier meets he finished second to Ryan Lochte in 100m fly at the Grand Prix at Mesa, Arizona, and won the 100m fly in Charlotte, North Carolina.
Phelps narrowly qualified for the 200m freestyle final in Charlotte, but opted not to swim it on the same night he won the 100m fly there.
Phelps is also slated to swim the 200m freestyle and 200m individual medley on the weekend.