Dominic Thiem serves in his semifinal singles match against Alexander Zverev at the ATP World Tour Finals on Saturday in London. Photo: VCG

World No.4 Dominic Thiem of Austria missed a chance to reach a career-best third in the ATP rankings with a Rio Open quarterfinal loss Saturday to Italy's Gianluca Mager.

Mager, a 128th-ranked qualifier playing in only his eighth career tour-level match, defeated 2017 Rio champion Thiem 7-6 (7/4), 7-5 in a match that was halted Friday by rain.

Had Thiem reached the semifinals in his 2020 clay-court debut, he would have overtaken Roger Federer for third in Monday's ATP rankings.

Thiem was down a set and a break when rain stopped the match Friday with Mager leading 7-6 (7/4), 2-1.

Mager served for the match in the ninth game of the second set but Thiem broke to pull ­level at 5-5, only to have the Italian claim the final two games and the match after one hour and 55 minutes on the court.

Mager booked a semi­final date against 106th-ranked Hungarian lucky loser Attila Balazs, who outlasted 133rd-ranked Spanish qualifier Pedro Martinez 2-6, 6-4, 6-2 in another rain-hit quarter-final.

Saturday's other semifinal pitted Chilean third seed Cristian Garin and fifth-seeded Borna Coric of Croatia, but rain again prevented those matches from being completed.

When the semifinals finally went on court late Saturday, Mager won the first set against Balazs 7-6 (7/4), and they were level at 3-3 when rain halted play again.

Garin was up 6-4, 4-4 on ­Coric in their match, with both to resume Sunday.