Former world number one Roger Federer, seeking a 21st Grand Slam crown, is seeded third for the tournament.
France's Paire, ranked 30th in the world, avenged a loss to India's Gunneswaran at Indian Wells in March.
Lee, ranked 212 in the world, turned pro at 15 and achieved his best ATP singles ranking of 130th in the world in 2017, toiling mainly on the developmental Challenger circuit.
The Japanese lost to 20th placed Sofia Kenin in the quarter-finals on Sunday due to a knee injury. Her participation at the US Open is still uncertain.
The 2017 US Open finalist notched her fifth career title and second this season after a win in Charleston in April.
Medvedev's upset win sent him into a title showdown with another unexpected finalist, 16th-seeded Belgian David Goffin, who defeated France's Richard Gasquet 6-3, 6-4 to reach his first ATP Masters 1000 final.
Barty could have re-taken the top ranking she lost last week to Japan's Naomi Osaka had she reached the final.
Barty has a chance to return to the world number one: but in the latest WTA calculation, Naomi Osaka and Karolina Pliskova also figure in the race for the top spot.
Djokovic was never in danger in his 6-3, 6-4 triumph over 53rd-ranked Spaniard Pablo Carreno Busta, whom he finished off in 90 minutes.
Eighth-seeded Khachanov rallied for a 6-7 (3/7), 7-6 (7/4) 6-2 victory over the volatile Australian, but the final set was close to farce, with Kyrgios engaged in a running dialogue with Irish chair umpire Fergus Murphy.
Australia's Barty, 23, won the battle of former number one players in just over 90 minutes.
Williams had hoped to rebound from the back spasms that forced her out of the WTA final in Toronto on Sunday, where she lasted only four games before retiring.
The 21-year-old lost to Serena Williams in the quarterfinals of last week's Rogers Cup, but benefitted from the second round defeat of former number one Ashleigh Barty and the failure of Karolina Pliskova to reach the semis in Toronto.