Tommy Haas failed to ride to the aid of fellow German-speaker Roger Federer, losing 6-4, 7-6 (7-0) Thursday to Rafael Nadal at the Cincinnati Masters and moving the Spaniard a step closer to the number-one ATP ranking, dpa reported.
Haas lost for the third time to Nadal, who can take the top spot from Federer by lifting the title on Sunday.
Nadal has been on a tear this season as he bears down on Federer, who has held the position for four and a half years.
Ivo Karlovic opened up the struggle a few hours earlier as he shocked Federer 7-6 (8-6), 4-6, 7-6 (7-5).
Federer saved a pair of match points in the third-set tiebreaker but could do little against a 22-ace attack by the 2.08-metre tall Croatian, whom he had beaten in all six previous meetings.
"When he serves like that, what can you do?" asked Federer, winner of only two minor titles this season and now sporting his most losses (11) since his 17 back in 2003, before his four-and-a-half-year run atop the ATP rankings.
"Maybe if I had won the first set, I'd be sitting here winner in two sets, but it was not to be. It was kind of brutal today."
It took just over two hours for Federer to fall and leave his top ranking at the mercy of Nadal. The Spaniard must still win the title Sunday at the Ohio event, or the Swiss will remain atop the rankings with the Beijing Olympics looming.
Federer could not rescue himself despite 48 winners and only 14 unforced errors. It is the only match in his career that he has lost despite never dropping serve.
"I guess I'll analyse and assess my game after the US Open. For the moment it's just all a blur," Federer said.
"It's so many big tournaments in a row. It's hard. It's a tough trip we've got up in front of us now, going back to China and coming back to the Open. But I'm looking forward for the next two tournaments."
Third-seeded Novak Djokovic takes a welcome dose of confidence into Friday's quarter-final with an old foe, after defeating Italy's Andreas Seppi 6-1, 6-2.
The Serb, who holds the Australian Open title, will face former Munich academy schoolmate Ernests Gulbis after the Latvian beat seventh seed James Blake for the second time in two months, 6-4, 1-6, 6-3.
Djokovic, 21, has won all three of his matches against Baltic teenaged talent Gulbis, most recently in May in Paris.
Spain's unseeded Carlos Moya completed a match interrupted by rain in the second set over Nikolay Davydenko, then defeated Igor Andreev 6-4, 7-6 (7-2).
On-form Andy Murray beat the heat over Dmitry Tursunov, 6-3, 6-3.
German Philipp Kohlschreiber out-duelled Swede Robin Soderling 7-6 (7-4), 5-7, 7-6 (10-8) in hot, humid weather.