Oddsmakers have Koepka as the best bet to win the tournament, with Thomas listed as the second favorite to win. Thomas won the 2017 PGA Championship before Koepka's back-to-back titles in 2018 and 2019.
"I feel like I've been playing better than my results since we came back from the quarantine," Thomas said Sunday after he won the FedEx St. Jude Invitational.
The 2020 PGA Championship starts Thursday and ends Sunday at TPC Harding Park in San Francisco. The tournament will air on ESPN and CBS, and players will compete for a total purse of $11 million, with nearly $2 million going to the winner.
Koepka is looking to become the first player to win the PGA Championship three consecutive times in the stroke-play era. Walter Hagen won four consecutive times in the match-play era from 1924 through 1927. Hagen is the only other golfer who won least three straight times.
Woods, Thomas and Koepka will tee off for the first round in the same group at 11:33 a.m. EDT Thursday in San Francisco.
No spectators will be allowed because of the coronavirus pandemic.
"Major championship weeks don't need anything special or spectators for us to know it's a major championship," Rahm told reporters Tuesday. "It just feels like it. Obviously fans will be missed. The atmosphere won't be the same."
Thomas should be Koepka's biggest challenge for a third consecutive title, but has said he needs to familiarize himself with the course, which he has never played.
Thomas trending up
The 2020 PGA Championship marks the first major championship since the July 2019 British Open, when Koepka tied for fourth.
Koepka finished tied for second at last week's FedEx St. Jude Invitational. Thomas has outplayed Koepka since the PGA Tour season resumed in June after play was suspended in mid-March due to the pandemic.
Koepka was No. 1 in the Official World Ranking at the end of 2019, but that spot now belongs to Thomas, who placed in the top 10 in four of the six tournaments since the season resumed. His latest run includes a second-place finish in July's Workday Charity Open.
Koepka has two top 10 finishes since the season resumed. He also has missed the cut twice and finished tied for 32nd and 62nd in six appearances. His last win came at the 2019 FedEx St. Jude Invitational.
The 2020 PGA Champions initially had been scheduled as the second major championship of the year from May 14 to 17 before it was moved to August due to the pandemic.
The Masters Tournament had been the first major championship on the schedule, but now will be the last, pushed to November from April.
Woods has won the PGA Championship more than any player in the field, with four titles among his 15 major triumphs.
Woods was hot at the end of 2019 and through January, with a win and three top 10 finishes in three tournaments, but has become less of a threat.
Woods finished 68th in January's Genesis Invitational before he returned to the PGA Tour for July's Memorial Tournament, where he nearly missed the cut before tying for 40th.
"I feel good," Woods said Tuesday. "I haven't played much competitively, but I've been playing a lot at home. I've gotten plenty of reps that way. This is what I've been gearing up for."
Woods missed the last two tournaments. He said poor putting and difficult conditions led to his tepid performance at the Memorial Tournament. Temperatures are expected to be in the mid to upper 60s, with winds up to 20 mph this week in San Francisco. No rain is expected.
The U.S. Open is the next major championship on the PGA Tour schedule. That tournament had been planned for June 18-21, but will take place Sept. 17-20 at Winged Foot Golf Club in Mamaroneck, N.Y.
The 2020 British Open has been canceled, while the 2020 Masters Tournament will be Nov. 12-15 in Augusta, Ga.
2020 PGA Championship
When: Thursday through Sunday
Where: TPC Harding Park in San Francisco
TV: First and second rounds, 4 to 10 p.m. EDT Thursday and Friday on ESPN; third round, 1 to 4 p.m. EDT Saturday on ESPN, 4 to 10 p.m. EDT Saturday on CBS; fourth round, noon to 3 p.m. EDT Sunday on ESPN, 3 to 9 p.m. EDT Sunday on CBS.
Streaming on ESPN+: First and second rounds, 10 a.m. to 4 p.m. EDT Thursday and Friday; first and second round featured groups, 10 a.m. to 10 p.m. EDT Thursday and Friday; third round, 11 a.m. to 1 p.m. EDT Saturday; third round featured groups 11 a.m. to 10 p.m. EDT Saturday; fourth round 10 a.m. to noon EDT Sunday; fourth round featured groups, 10 a.m. to 9 p.m. EDT Sunday.