Australia's Ashley Bartyis eyeing top spot. Photo: Tim Goode/PA via AP

LONDON Ashleigh Barty is one game away from regaining the world number one spot after she beat Greece's Maria Sakkari 5-7, 6-2, 6-0, and Naomi Osaka retired injured on Friday in the Cincinnati quarter-finals. 

Meanwhile, world number one and defending champion Novak Djokovic is through to the semi-finals with a 7-6 (7-2), 6-1 win over Frenchman Lucas Pouille, despite complaining about his right elbow. 

This was the elbow Djokovic previously had surgery on at the start of 2018 before he returned to win three grand slams in a row at Wimbledon, the US and Australian Open. 

Up next for Djokovic in the final four is Russian Daniil Medvedev, who bested his 21-year-old compatriot Andrey Rublev 6-2, 6-3. 

The 23-year-old Barty lost her top ranking to Osaka after an early exit against Sofia Kenin in Montreal last week. 

Osaka played Kenin in the last eight but had to retire with a left knee injury when she was trailing the 20-year-old American 6-4, 1-6, 2-0.  

"She felt bad. I just wish her a speedy recovery," Kenin commented on Osaka's injury. 

"I'm just finding my rhythm while playing really well. I couldn't be happier with how things are going."

Barty will play two-time grand slam champion Svetlana Kuznetsova in Saturday's semi-finals after the Russian eliminated world number three Karolina Pliskova in a 3-6, 7-6 (7-2), 6-3 victory.  

If she beats Kuznetsova, Barty will automatically be reinstated as world number one. 

Kenin, who makes back-to-back semi-finals following Montreal, goes on to face fellow American Madison Keys, who surpassed Venus Williams 6-2, 6-3. 

Richard Gasquet made the semi-finals with a 7-6 (7-2), 3-6, 6-2 win over Spain's Roberto Bautista-Agut. 

"I know how tough it was to come back," said Gasquet, who was absent for the first six months of the tour due to groin surgery in January.

"I know the moments I had at the start of the year, so I just wanted to enjoy, to fight." 

The 34-year-old Frenchman will play Belgium's David Goffin next after Japan's Yoshihito Nishioka pulled out of their contest with illness from food-poisoning. 

Barty almost let her match with Sakkari slip when the French Open champion was serving for the first set at 5-3. She faltered through errors as Sakkari stole the lead in game 12. 

"I started the match well and I went away from what was working. Maria took some risks that paid off," said Barty. 

"It was important for me at the start of the second set to go back to basics and what worked before. I tried to change the tempo of the match." 

Barty powered through the second set and capped the equalizer with a forehand down the line after making only four unforced errors in the frame. 

The former world number one kept her hopes alive of retaining the position as she raced through six straight games to take the match in 105 minutes. 

Djokovic could have broke Pouille's serve in the opening game but his French opposite held on for a tie-break. 

Pouille made a combination of errors, including a double-fault, to fall 6-1 behind in the tie-breaker before Djokovic wrapped up the first set by punching a cross-court backhand winner. 

In the second set, Djokovic surged 4-1 in front but during the changeover, he called for treatment on his right elbow. 

The 16-time grand slam winner was given a break at 5-1 to serve for the match, and Pouille netted for one last time to see Djokovic through to his 65th Masters semi-final. 

Kenin got off to a good start against Osaka as she broke serve first to lead 3-1 and held serve to see out the first set. 

The two-time grand slam champion Osaka fired back with two breaks to level the match, but ran into an injury at the start of the third set. 

After Kenin held serve for 1-0, Osaka called for a medical timeout at 40-40 in her following service game. 

The 21-year-old complained about discomfort in her left leg, and asked for her knee to be taped. 

Osaka resumed serving but moments later, she walked to the umpire's chair and conceded defeat by being unable to continue.