Top seed Roger Federer struggled to a 6-3, 3-6, 6-2 victory over Spaniard Nicolas Almagro on Friday to reach the semi-finals of the Cincinnati Masters.
The world number one was unusually erratic as he dropped his first set of the week before recovering his poise to set up a clash with Lleyton Hewitt.
"I actually think I played better today than I did yesterday," Federer told reporters.
"The only problem was I had one really bad game which cost me the second set.
"I still thought my serving was good today and I hit better from the baseline."
Federer, looking for his 50th career title, broke in the sixth game but immediately dropped his serve before breaking for a second time to take the opening set.
Almagro, appearing in his sixth quarter-final of the year, broke serve in the eighth game of the second set to level the match before an early break in the decider was enough to take Federer through.
COMMANDING PERFORMANCE
Hewitt, twice a runner-up here, produced a commanding performance to beat fellow former world number one Carlos Moya 6-2, 6-4 and reach his fourth semi-final of the season.
World number 19 Moya had defeated fourth seed Novak Djokovic of Serbia earlier this week but the Spaniard made too many unforced errors against Hewitt.
"I am playing well at the moment," the Australian told reporters. "I thought my court coverage was exceptional today.
"But it's hard to close him out, he's such a strong player."
Federer has won his previous 10 meetings with Hewitt, including a straight-sets victory last week in Montreal.
"I don't think the nine in a row mattered much before last week because we hadn't played for about two years," said the Swiss. "In two years a lot of things happen.
"It's good I played him last week so I've got an idea of how he plays."
The second semi-final will be between fifth seed Nikolay Davydenko of Russia and American ninth seed James Blake.
Davydenko withstood a late fightback from 16th seed David Ferrer to beat the Spaniard 6-2. 6-4.
The Russian led 4-0 in the second set before Ferrer cut the deficit to 4-3 but Davydenko held on for victory.
Blake had to battle hard to beat his compatriot Sam Querrey 5-7, 6-4, 6-4 to reach only his second Masters Series semi-final.
"I felt like I did a great job mentally after losing that first set and not playing my best," Blake said after seeing off the wildcard.
"Just hanging with it, still serving well, and I didn't get broken after that and still going after my shots but they were going in."