Frost parred the final two holes to hold on for a one-stroke victory over Couples on Sunday in the Regions Tradition, the South African's first major title in 17 attempts on the Champions Tour.
"We came in from the break and I said to myself, 'It's not going to be easy,'" Frost said. "Freddy's got nothing to lose. He's going to go out there and hit the shots and fire at the pins. I've just got to kind of accept it and be up to the challenge."
Both birdied No. 16 after play resumed with the sun shining after a delay of 1 hour, 8 minutes for lightning in a round played at times in driving rain.
Couples and Frost both shot 4-under 68, and Frost finished at 16-under 272.
It was Frost's second victory of the season and fifth career win on the 50-and-over tour.
"We both played very hard today. I don't know who wanted it more, me or him," Frost said. "He's just such a tough competitor. I didn't think I had it in me, all the good shots he hit out there. I just had to try to stay on top of him."
The two bumped fists coming up on 18 in what had ultimately turned into a two-man duel.
"He played very well," Couples said. "I gave him pretty much everything I had. I hit some pretty good shots. I drove it pretty well. He was just better the whole day. He stayed one ahead and that's where we ended up."
Also the Toshiba Classic winner in March in California, Frost hit his approach shot to 10 feet of the closing hole and two-putted to match Couples' par.
Couples had sent his approach into the bunker at right and his potential birdie putt went just to the left.
"I said to myself, 'If he makes his (putt), I'm just going to have to make mine.' I've been in this match play situation many times before and that's just how I try to envision what's going on," Couples said. "You can't help it if he's going to roll his putt in."
The next round of heavy rain started within a couple of minutes of the end of the round, which opened in the morning with a two-tee start.
Cook closed with a 66 and Toledo had a 67.
Couples was seeking a second straight win in a major. He sat out the Senior PGA Championship three weeks ago but won the Senior Open Championship in 2012.
Frost had come close in a major before but this time managed to nurse his one-stroke lead through the final round. He lost to Tom Lehman in a three-way playoff at the 2010 Senior PGA Championship.
Frost's only bogey came on No. 14 while Couples got his 68 with six birdies and two bogeys.
Couples, who had a 3-footer for birdie on No. 15, left the next hole with a grin after matching Frost's birdie.
His expression turned to a wince when his second shot on No. 17 went far to the right, landing down a steep slope from the green next to the grandstands.
He two-putted for his fourth straight par on the hole and couldn't make up that stroke.
"Really, the letdown, I hit a terrible shot on 17," Couples said. "I smoked it down there and I just tried a shot that maybe I'm not accustomed to. Under that kind of pressure, I think it was a mental blunder to not hit the green there and not make birdie."
Cook had beaten Frost in a playoff at the season-opening Mitsubishi Electric Championship in Hawaii, and made a final-round run at Shoal Creek.
Cook birdied five of the first eight holes and finished his round with a 66 a couple of minutes before the weather delay.
"We were running all day trying to outrun it," Cook said. "I guess this is the very definition of scattered thunderstorms. They were all over the place."
Langer, the points leader, closed with a 63 to tie the course record with Gary Player from the 1984 PGA Championship. Langer had 10 birdies and finished at 10 under.
"If I had made some putts earlier in the week," he said, "it would have been pretty nice."
Two-time defending champion Lehman tied for 22nd at 6 under.