Estes took his share, recording seven birdies without a bogey, five of them after his turn on Harbor Shores' front nine. He birdied No 1, 3, 5, 6 and 9 on his march to the clubhouse.
"I love a clean card," said Estes, whose only top-10 finish this season was a solo fourth at the Rapiscan Systems Classic, which Alker won April 4.
"I made some good saves on both sides."
Like the previous four Senior PGA Championships played at Harbor Shores, this season's second major for the 50-and-older crowd is shaping into a low-scoring affair. 56 golfers in the 156-player field shot rounds of one-under or better on a cloudy day after an overnight rain softened the course. More rain and cooler temperatures are expected for Saturday's round.
On the European Tour, Kiwi Ryan Fox is in a share of 29th at the Dutch Open.
A solid finish at the tournament will likely lock in a trip to the US Open for Fox, who mixed five birdies with a bogey and a double bogey in a two-under round.
He is five shots off the lead held by Joost Luiten, with Eddie Pepperell and Rasmus Hojgaard one shot off the pace.
Fellow Kiwi Danny Lee also had a double bogey in his first round at the PGA Tour's Charles Schwab Challenge, but didn't have as many birdies as Fox.
Three bogeys, two birdies and the double saw Lee sign for a three-over round, sitting in a share of 91st, seven shots off the lead held by eight players including major winners Patrick Reed, Webb Simpson and Scottie Scheffler.
- With AP