C172 pilot reported that the left tank fuel pressure became unported during a forward slip; causing fuel starvation and leading to a loss of engine power.

Narrative

I arrived at ZZZ to do a brief flight. Preflight checks were completed and no concerns were observed. I started the airplane and performed a short warm-up and began a taxi to the Runway XX run-up area. Before takeoff; the engine run-up and pre-takeoff checklist was followed and completed. There were no concerns observed and the engine and all systems were performing as specified by the aircraft operation manual. I proceeded with the flight and departed the pattern to the south. I climbed to 3000 ft. On the southern portion of ZZZ; I turned the airplane around and lined up with Runway XX to fly an approximate 3-mile final. My altitude was very high for the given distance; so I performed a forward slip to lose altitude. I had not yet performed my pre-landing checklist and the airplane was still being operated on the left tank. During the forward slip; I applied right rudder and left aileron. The AWOS in ZZZ was not working and the data was also not accessible through aviationweather.gov - has been this way for several days - so I used the METAR data from ZZZ1 before takeoff; which showed wind at 010 at 7 kt. Prior to takeoff the windsock was still; but pointed directly down the runway favoring Runway XX; my personal observation is that wind was calm. Once into the forward slip; the engine began losing power and the RPM reduced to 700 RPM. I discontinued the forward slip and pitched the airplane for best glide. I began performing flow checks. I pulled carb heat; verified the mixture was full rich. I checked the magneto switch and also observed no failed magneto indication on my digital tachometer.I primed the engine; but did fail to switch tanks to both on the fuel selector. My thinking at this point was that I was rapidly approaching the airport environment and questioning if I was able to reach the runway. So; my focus switched to flying the airplane and landing. I made the airport property and landed the aircraft safely. I observed no damage after the landing. The airplane started and I drove it to the hangar without incident. During the preflight I checked the levels of my fuel tanks. I used a 'Fuelhawk' fuel gauge to check the fuel tank levels. There were 7 gallons usable fuel in the left tank and 6 gallons usable in the right tank; enough for nearly 1.5 hours of operation. I believe that the left tank fuel pressure became unported during the forward slip; causing fuel starvation. I learned and in the future will remind myself to ensure that I have the fuel selector set to 'both' before performing a forward slip.