(Reuters) - Rafa Nadal and Stan Wawrinka, two of the headliners at the Western & Southern Open, fell in singles to a pair of young guns on a hot, steamy day on the hardcourts in Cincinnati.

Third-seeded Nadal, showing fatigue following his play at the Olympics that included a gold medal in doubles, was a step slow and paid the price against 19-year-old Borna Coric of Croatia, who reached the quarter-finals with a 6-1 6-3 win.

Grigor Dimitrov scored another upset when he eliminated sluggish second seed Stan Wawrinka, the Bulgarian making the most of a single break of serve in each set to win 6-4 6-4 and book his berth in the quarters.

Australian Bernard Tomic, 23, notched yet another upset with a 7-6(1) 7-6(5) win over fifth-seeded Kei Nishikori of Japan, who had won the bronze medal match in Rio over Nadal.

Coric advanced to his first ATP World Tour Masters 1000 quarter-final, dominating the 14-times grand slam winner by reeling off 10 straight games to claim the opening set and charge into a 4-0 lead in the second set.

Nadal received treatment on his left wrist after the first set and continued to fight, but struggled against the weight of Coric’s ground strokes and strong serve.

"It was an unbelievable match on my side. I think he wasn't playing his best tennis," said Coric, who saved a match point against Australian Nick Kyrgios in a third-set tiebreak on Wednesday before prevailing.

The Spaniard rallied at the end before a relieved Coric closed it out on his second match point with a backhand winner.

"I was thinking, 'Rafa please miss, I cannot make a winner'," he said of Nadal's fightback.

Next up for Coric is the winner of the match between sixth seed Tomas Berdych and 12th-seeded Marin Cilic.

Dimitrov, 25, who has struggled with inconsistency, was buoyed by his performance against doubles partner Wawrinka.

"I need matches like that," said Dimitrov. "I need 10 or 12 more of those matches to sustain that level and also to send a statement not only for me, but towards everyone."

Dimitrov, who improved his record to 4-2 against Wawrinka, was near flawless on serve in the second set, dropping just three points.

The Bulgarian will next face American Steve Johnson, who saved four set points as he advanced to the quarters over seventh seed Jo-Wilfried Tsonga of France 6-3 7-6(6).

Johnson, who claimed a third-round win over Dimitrov this year at Wimbledon, boasts a 20-7 record since June.

(Reporting by Larry Fine in New York; Editing by Andrew Both)