Nadal, the world No. 1, defeated No. 6-ranked Tomas Berdych 6-4, 1-6, 6-3. That gave Nadal a 3-0 record in Group A play and dropped Berdych to 1-2.
Had Berdych beaten Nadal, he would have qualified for the tournament semifinals. After the strong second-set showing, that appeared possible, but with a break in the eighth game of the third set, Nadal was able to extend his personal winning streak versus Berdych to 16 matches and eliminate Berdych.
Wawrinka, No. 8 in the rankings, had a 2-1 group record after he beat third-ranked David Ferrer 6-7 (3-7), 6-4, 6-1 earlier Friday. He then had to wait out the Nadal-Berdych match to see if he would be playing Sunday.
By winning the first set Friday, Nadal locked up the No. 1 spot in Group A. He will go against the winner of Saturday's Group B match between No. 5-ranked Juan Martin del Potro and seventh-ranked Roger Federer in the semifinals. Wawrinka takes on world No. 2 Novak Djokovic, already assured to be the winner of Group B, in the other semifinal.
Sunday's winners meet Monday for the title.
Nadal had two breaks Friday -- the first and last games Berdych served. The first break won the first set for Nadal but Berdych roared out to a 5-0 lead in the second set with a pair of breaks of his own, leaving his and Wawrinka's fate to the third set.
Neither player had a break chance until Nadal, helped by a Berdych double fault, moved ahead 40-0 in the eighth game. Berdych won the next point but again had a double fault, handing the break to Nadal. Five points later, the match was over.
Ferrer, even though without a chance to make the semifinals, often played spirited tennis Friday. He rallied in the first set after Wawrinka went up 5-2 and was serving for the set and eventually got ahead 6-5. Wawrinka did break Ferrer at love and forced the tiebreaker but Ferrer jumped out to a 4-0 lead before collecting his first set of the tournament.
Ferrer opened the second set by holding serve at love and had a break point against Wawrinka in the second. Wawrinka, though, was able to eventually hold and then posted a break for a lead that held through the rest of the set.
Wawrinka opened the third set with a break. Ferrer held to get to 1-2 but Wawrinka swept the rest of the games, finishing his sixth break and the match with a backhand winner.
Round-robin play finishes up Saturday, when del Porto takes on Federer in the day match. Djokovic, 2-0 in the group, goes for the round-robin sweep when he plays No. 9 Richard Gasquet, 0-2 and unable to advance, in the late Saturday match.